That is not legally possible in the EU. You can grant irrevocable usage rights, but you cannot give away your copyright.

Most Terms of Service don't do that, instead asking you to provide a "perpetual" "irrevocable" "transferable" license for your content -- and while some absolutely stretch the terms to allow them to use it for things like language model learning or shifty monetization practices, such a license is also legally necessary for the website to function at all.

For "open-source" websites like Wikipedia or OSM, the terms are usually even simpler - you agree to license your posts under the same license that they use to distribute it.

As for Fandom specifically, they seem to mostly operate on the latter model -- though you still need an additional commercial use waiver if you want to submit to NC or ND-licensed wikis (which once again goes into the "legally necessary" box).

The same open-source license that lets people edit the wikis and fork them to independent websites without having to ask permission from every single contributor also lets Fandom admins reject attempts to delete or redirect pages.