Brave and Opera both use their own solutions for syncing.

In fact, can anyone name a browser except Chromium that uses Chrome's sync servers for syncing? Because I can't.

Also your point about backwards compatibility doesn't really make sense as Google has plenty of ways to detect browser version and capabilities of the sync engine (the sync engine sends that info to the sync server voluntarily right now).

Also Google can't just disable it's own API keys every time there is an update because this would shut down a large number of it's own users too. They would need to wait at least 1-3 months to avoid larger disruptions and this would give others more than enough time to fetch the latest keys (and if there is interest, there will always be someone who does that and posts them).