Here is a refreshed code for 1.17 and 1.18+. This version no longer support 1.16, but have a far more optimized code.
New option : AutoEnableNinjas. Default : enable (player are ninjas when joining your server). You may also want to decide manually if player are ninjas or not using :
/scoreboard players set PLAYER_NAME ninjam.enable 1
/scoreboard players set PLAYER_NAME ninjam.enable 0