Hi, in my community we had Discussions enabled but I also tried using the Forums as well, and just today I saw that it has now officially been replaced and the old topics have carried over.
I'm not sure if its the same for other Wikis. Other Wikis that didn't have Discussions enabled still have their Forums.
I did ask for the feature to be enabled and knew the possibility that Forums will eventually be replaced, but now that I'm stuck with it I'm realizing how it's not so pretty looking, I would like it if the categories were physically separated but still on one page, right now it looks disorganized.
Unfortunately, removal of Forums was one of the conditions for switching to Discussions; if I recall correctly. As far as we know, Wikia still intends to force all communities onto Discussions at some point in the future. However, as you have noted, this has not yet happened for older communities that have not opted in.