qmk_firmware/docs/ChangeLog/20190830/PR5176.md

• Migrating ACTION_LAYER_MOMENTARY() entries in fn_actions to MO() keycodes
  • fn_actions is deprecated, and its functionality has been superseded by direct keycodes and process_record_user()
  • The end result of removing this obsolete feature should result in a decent reduction in firmware size and code complexity
  • All keymaps affected are recommended to switch away from fn_actions in favour of the custom keycode and macro features