User blog comment:Cristina7/Introducing Discussions, a New Platform For Engaging Your Community/@comment-452-20161104233022

Apparently the new forum has been removed from wikifeatures entirely - even on wikis where it is enabled, so it is now impossible to disable it.

I can find nothing about this change in any technical update.

On new wikis, Discussions is enabled by default instead of Forums, so apparently Wikia Staff regard it as "finished".