I don't agree with this sentiment at all, to be honest. In my opinion, the ultimate fate of every extension is to be rolled into the standard or to be relegated to the bins of history. The idea of having various levels of languages based on how advanced they are perceived to be sounds extraordinary to me (also quite a bit patronising).
Generally speaking, you opt in to a feature by using it. GHC2021 should not have features that we, as a community, don't recommend using (the bins-of-history ones); but, surely, TypeFamilies is not one of these. It is not to say that GHC2021 should have TypeFamilies, but that them being perceived as advanced is not, in my opinion, a relevant criterion. We should be asking, instead, whether it is ready.