That’s what the original mod for switch is called, soft mod.
By restarting you either drop into a “launcher” that gives you various options including booting into the original OS or your modded OS.
I set mine up so if it restarts it always goes into the modded OS. Many people have theirs set up to go to the launcher, instead. People usually boot into the original OS if they want to play online.
LineageOS 21 now officially supports the Nintendo Switch ( 9to5google.com )