Saving BL Touch settings
From this source you can read:


  After a G29 the leveling data is only stored in RAM. You have to use M500 to save the bed leveling data to EEPROM, otherwise the data will be lost when you restart (or reconnect) the printer. Use M502 to reset the bed leveling data (and other settings to defaults). Use M501 to reload your last-saved bed leveling from EEPROM. This is done automatically on reboot.


The source also answers the use of M420 S1:


  After a G29 bed leveling is automatically enabled, but in all other situations you must use M420 S1 to enable bed leveling. It is essential to include the command M420 S1 in the “Start G-code” in your slicer settings. If you have no bed leveling, or if there is no leveling data, then this command is simply ignored.


So, if you're not using a G29 in your start code you must use the G-code  M420 S1 to enable the stored mesh from memory.