As long as you’re online and connected to FLS, nothing is missing from Single-Player. Except, of course, the fact that the game world is running only around the player, so e.g. meteor showers don’t trigger unless the player is in the vicinity.

If you’re not connected to the FLS, you won’t have access to the Battle Pass, the Black Lotus Bazaar, or the challenge system. Items and recipes etc. gained via those systems are unaccessible. Everything else should still work just fine.