Do experience counters remain when the card creating them leaves the battlefield?
Yes, you keep the experience counters if Mizzix leaves the battlefield.

Mizzix's ability says


  Whenever you cast an instant or sorcery spell with converted mana cost greater than the number of experience counters you have, you get an experience counter.


The counters go on you, the player, rather than on any other object. When creatures die that has no effect on counters on players.