I know you can get an enderres on Endertopia, but can you visit it if you're no longer a supporter? I know you keep your normal reses, but I also know that only supporters can visit Endertopia. So, can non-supporters visit enderreses?
You must maintain supportership in order to access the endertopia. Therefore, no, you can not access it being non-supporter.
Endertopia is still being used. A few months back I wondered about it myself as well, and mentioned this within the contribution team. It's still being used and some players are still getting themselves their own apartment. The thing not to overlook (which I did): Endertopia is more than merely the Enderman XP farm. People already answered, but I'd like to complete it: on Utopia the Waste & Frontier can only be used by supporters (gold and up). Endertopia is in the Frontier End, therefor it's not possible to visit this if you're not gold or diamond. It is not a full residence like those in town, but mostly an "apartment" where you get yourself several storage options. You can learn more about this on the Endertopia wiki page.