It auto turns itself off at 60. You can turn it back on at that point, although in a roundabout way by going through the dialog options. I’ve had it turn itself back off on an account with Dragonflight. (I’m not sure what triggers it. Just logging in on one character turned it off, but Udiza weirdly enough got to 61 in Dragonflight content and then got kicked out — I’d forgotten it was turned on and was a bit annoyed for the surprise port.) It will permanently kick you out at 61.
So, short version, if you don’t have the Dragonflight expansion, you can get away with out locking. If you do have it, lock at 59 or you’ll get kicked out at 61 and have to deal with it randomly kicking you out while you’re 60. (You can’t turn off XP anymore once you hit 60.)