monitor_baud => monitor_speed

platformio.ini

 # ATmega2560
 #
 [env:megaatmega2560]
-platform     = atmelavr
-framework    = arduino
-board        = megaatmega2560
-build_flags  = ${common.build_flags}
-board_f_cpu  = 16000000L
-lib_deps     = ${common.lib_deps}
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = atmelavr
+framework     = arduino
+board         = megaatmega2560
+build_flags   = ${common.build_flags}
+board_f_cpu   = 16000000L
+lib_deps      = ${common.lib_deps}
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 #
 # ATmega1280
 #
 [env:megaatmega1280]
-platform     = atmelavr
-framework    = arduino
-board        = megaatmega1280
-build_flags  = ${common.build_flags}
-board_f_cpu  = 16000000L
-lib_deps     = ${common.lib_deps}
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = atmelavr
+framework     = arduino
+board         = megaatmega1280
+build_flags   = ${common.build_flags}
+board_f_cpu   = 16000000L
+lib_deps      = ${common.lib_deps}
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 #
 # Due (Atmel SAM3X8E ARM Cortex-M3)
 #  - RADDS
 #
 [env:DUE]
-platform     = atmelsam
-framework    = arduino
-board        = due
-build_flags  = ${common.build_flags}
-lib_deps     = ${common.lib_deps}
-lib_ignore   = c1921b4
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = atmelsam
+framework     = arduino
+board         = due
+build_flags   = ${common.build_flags}
+lib_deps      = ${common.lib_deps}
+lib_ignore    = c1921b4
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 [env:DUE_USB]
-platform     = atmelsam
-framework    = arduino
-board        = dueUSB
-build_flags  = ${common.build_flags}
-lib_deps     = ${common.lib_deps}
-lib_ignore   = c1921b4
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = atmelsam
+framework     = arduino
+board         = dueUSB
+build_flags   = ${common.build_flags}
+lib_deps      = ${common.lib_deps}
+lib_ignore    = c1921b4
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 [env:DUE_debug]
 # Used when WATCHDOG_RESET_MANUAL is enabled
-platform     = atmelsam
-framework    = arduino
-board        = due
-build_flags  = ${common.build_flags}
+platform      = atmelsam
+framework     = arduino
+board         = due
+build_flags   = ${common.build_flags}
   -funwind-tables
   -mpoke-function-name
-lib_deps     = ${common.lib_deps}
-lib_ignore   = c1921b4
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+lib_deps      = ${common.lib_deps}
+lib_ignore    = c1921b4
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 #
 # NXP LPC1768 ARM Cortex-M3
   TMC2130Stepper@>=2.2.1
 extra_scripts   = Marlin/src/HAL/HAL_LPC1768/lpc1768_flag_script.py, Marlin/src/HAL/HAL_LPC1768/upload_extra_script.py
 src_filter      = ${common.default_src_filter}
-monitor_baud    = 250000
+monitor_speed   = 250000
 
 #
 # LPC1768 (for debugging and development)
   https://github.com/MarlinFirmware/U8glib-HAL/archive/dev.zip
 src_filter     = ${common.default_src_filter}
 extra_scripts  =  Marlin/src/HAL/HAL_LPC1768/debug_extra_script.py, Marlin/src/HAL/HAL_LPC1768/lpc1768_flag_script.py
-monitor_baud   = 250000
+monitor_speed  = 250000
 debug_tool     = custom
 debug_server   =
   C:\Program Files (x86)\SEGGER\JLink_V618d\JLinkGDBServerCL.exe
 # Melzi and clones (ATmega1284p)
 #
 [env:melzi]
-platform     = atmelavr
-framework    = arduino
-board        = sanguino_atmega1284p
-build_flags  = ${common.build_flags}
-upload_speed = 57600
-lib_deps     = ${common.lib_deps}
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = atmelavr
+framework     = arduino
+board         = sanguino_atmega1284p
+build_flags   = ${common.build_flags}
+upload_speed  = 57600
+lib_deps      = ${common.lib_deps}
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 #
 # Melzi and clones (Optiboot bootloader)
 #
 [env:melzi_optiboot]
-platform     = atmelavr
-framework    = arduino
-board        = sanguino_atmega1284p
-build_flags  = ${common.build_flags}
-upload_speed = 115200
-lib_deps     = ${common.lib_deps}
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = atmelavr
+framework     = arduino
+board         = sanguino_atmega1284p
+build_flags   = ${common.build_flags}
+upload_speed  = 115200
+lib_deps      = ${common.lib_deps}
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 #
 # RAMBo
 #
 [env:rambo]
-platform     = atmelavr
-framework    = arduino
-board        = reprap_rambo
-build_flags  = ${common.build_flags}
-board_f_cpu  = 16000000L
-lib_deps     = ${common.lib_deps}
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = atmelavr
+framework     = arduino
+board         = reprap_rambo
+build_flags   = ${common.build_flags}
+board_f_cpu   = 16000000L
+lib_deps      = ${common.lib_deps}
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 #
 # Sanguinololu (ATmega644p)
 #
 [env:sanguino_atmega644p]
-platform     = atmelavr
-framework    = arduino
-board        = sanguino_atmega644p
-build_flags  = ${common.build_flags}
-lib_deps     = ${common.lib_deps}
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = atmelavr
+framework     = arduino
+board         = sanguino_atmega644p
+build_flags   = ${common.build_flags}
+lib_deps      = ${common.lib_deps}
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 #
 # Sanguinololu (ATmega1284p)
 #
 [env:sanguino_atmega1284p]
-platform     = atmelavr
-framework    = arduino
-board        = sanguino_atmega1284p
-build_flags  = ${common.build_flags}
-lib_deps     = ${common.lib_deps}
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = atmelavr
+framework     = arduino
+board         = sanguino_atmega1284p
+build_flags   = ${common.build_flags}
+lib_deps      = ${common.lib_deps}
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 [env:STM32F1]
-platform     = ststm32
-framework    = arduino
-board        = genericSTM32F103RE
-build_flags  = !python Marlin/src/HAL/HAL_STM32F1/stm32f1_flag_script.py
+platform      = ststm32
+framework     = arduino
+board         = genericSTM32F103RE
+build_flags   = !python Marlin/src/HAL/HAL_STM32F1/stm32f1_flag_script.py
   ${common.build_flags}
-lib_deps     = ${common.lib_deps}
-lib_ignore   = U8glib-HAL
+lib_deps      = ${common.lib_deps}
+lib_ignore    = U8glib-HAL
   c1921b4
   libf3c
   lib066
   Adafruit NeoPixel
   libf3e
   TMC26XStepper
-lib_ldf_mode = 1
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+lib_ldf_mode  = 1
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 [env:STM32F4]
-platform    = ststm32
-framework   = arduino
-board       = disco_f407vg
-build_flags = ${common.build_flags} -DUSE_STM32GENERIC -DMENU_USB_SERIAL -DMENU_SERIAL=SerialUSB
-lib_deps    = ${common.lib_deps}
-lib_ignore  = Adafruit NeoPixel, c1921b4, TMC2130Stepper
-src_filter  = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = ststm32
+framework     = arduino
+board         = disco_f407vg
+build_flags   = ${common.build_flags} -DUSE_STM32GENERIC -DMENU_USB_SERIAL -DMENU_SERIAL=SerialUSB
+lib_deps      = ${common.lib_deps}
+lib_ignore    = Adafruit NeoPixel, c1921b4, TMC2130Stepper
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 #
 # Teensy++ 2.0
 lib_deps      = ${common.lib_deps}
 lib_ldf_mode  = deep+
 src_filter    = ${common.default_src_filter}
-monitor_baud  = 250000
+monitor_speed = 250000
 
 #
 # Teensy 3.5 / 3.6 (ARM Cortex-M4)
 #
 [env:teensy35]
-platform     = teensy
-framework    = arduino
-board        = teensy35
-build_flags  = ${common.build_flags}
-lib_deps     = ${common.lib_deps}
-lib_ignore   = Adafruit NeoPixel
-src_filter   = ${common.default_src_filter}
-monitor_baud = 250000
+platform      = teensy
+framework     = arduino
+board         = teensy35
+build_flags   = ${common.build_flags}
+lib_deps      = ${common.lib_deps}
+lib_ignore    = Adafruit NeoPixel
+src_filter    = ${common.default_src_filter}
+monitor_speed = 250000
 
 [env:malyanm200]
 platform    = ststm32