Help me understand what this means to e.g. GrapheneOS, please. Will it be able to exist and just have to wait longer for updates or will it be in real jeopardy?
Potentially longer gaps between major updates, since they can no longer follow upstream closely. However, Google might have planned to slowly make Android proprietary, which means death to 3rd party forks.
The big problem is that aaaaaalll these forks are still just a tiny tiny tiny drop in the bug mobile phone OS bucket.
If the forks want to be sustainable they need to cater to the market a bit. (Of course that's much harder said than done, but we see - for example with Nothing Tech - that there are new successful upstarts from time to time.)