It would take some clever crafting to outlaw Matrix clients without also outlawing web browsers and conventional email clients. Let's assume they did though. The best they can do is block it from app stores, which won't stop anyone but iOS users.
More likely, it just won't become popular enough for lawmakers to notice because the UX is a little rough, and people have very little patience for such things anymore.
Google has backed away from that, stating that an "advanced workflow" with more warnings than the current settings toggle will remain available. We should all be concerned they even considered such a thing though.
(Non-Android) Linux phones often aren't GNU; PostmarketOS is one of the more popular options, and that's based on Alpine Linux which uses musl and Busybox.
More likely, it just won't become popular enough for lawmakers to notice because the UX is a little rough, and people have very little patience for such things anymore.