I (could be wrong but) highly doubt it will be removed. It cannot remain a default because it's unintuitive and initially confusing, and GNOME is strongly focused on simplicity and new users. But the designers are well aware that failure to provide the classic behavior (my guess: probably through a toggle in Tweak Tool) would disappoint tons of users.
Which is not to say that GNOME doesn't have a feature-removal problem; I just don't think middle-click paste specifically is going to wind up a victim of it. My guess is that you'll discover another actually-removed feature to actually miss.