"We were discussing this in today's Triage meeting. Our current plan is to turn Metal on soon. This will increase the size of our engine by ~200KB. We can eventually reduce this by removing the OpenGL code but (a) that would drop support for <iOS10, which we don't want to do anytime soon (Skia only does Metal from iOS10), and (b) would require some work from the Skia team (OpenGL can't currently be removed from the codebase)."
I am really curious why you are concerned at all about supporting iOS 9 (and lower?) at this point. That's a tiny, tiny fraction of worldwide iOS users. Probably not more than 1%. How many people are creating apps in Flutter that need to target that far back, and that user base?
Seems like it will be solved 'soon'