Maybe leave the legacy version on Itch. So prople can still play it, but the full experience is steam only.
And seeing as some countries like Vietnam don't have steam but DO have itch, it means you took the game away from those people and they can no longer play it, which is kind of a jerk move.
Also the fact is, the browser version worked on mobile, shokingly well too. So to throw away that mobile support feels silly.