After upgrading to 1.16 I get a warning about "experimental settings"

The warning is indeed caused by enabling snapshots. Snapshots are intermediate releases that aren't tested properly. A snapshot may contain game breaking bugs like accidentally regenerating chunks in your worlds. It might be highly unstable, where certain new mobs may crash your game. Like, hard crash to desktop because an error occured that Java is unable to deal with. A snapshot might have basic things like jumping or mining broken. And there is no guarantee that things you've done in older versions still work in the snapshot version.

Snapshots are meant for enterprising players who want to play with the newest of the new content and aren't bothered if a bug ends up corrupting their world or destroying hours of their work. They just clarified that in 1.16 so people who may have accidentally enabled them are aware that they're using experimental settings that may end up ruining their world.