Can I still play on after activating FLAG is END in Baba is You?

I can see that in 'A Way Out?' I can win the level with WIN but I can also make the flag END. I want to do END but I can tell I have much more content left and I don't want to get locked out of my 30+ hour save file. It seems to be impossible to tell if this is the case without running into spoilers, since I don't want to see the ending before I beat the game. If I activate FLAG is END will I be able to keep playing on that save file?

Solution 1:

Yes!

I tested it by copying my save file and getting the ending and after the credits I could still play on that save file.

Solution 2:

Yes. You can always go back to a level that you've previously been able to reach. This is true whether you win the level or do anything else that might cause you to exit the level.

The only things that can't be undone are consequences of winning a level: you can't re-lock any levels that it unlocked, un-mark the level as won, reduce your win count...

In the late game, (mild spoiler — vague wording that doesn't actually reveal any surprises)

there are times when it's not trivial to return to a prior level. But it's always possible. This thread explains the mechanics of one such case.