[beautron]

> And it doesn't really color the function because you can trivially make it sync again.

Yes, but this goes both ways: You can trivially make the sync function async (assuming it's documented as safe for concurrent use).

So I would argue that the sync API design is simpler and more natural. Callers can easily set up their own goroutine and channels around the function call if they need or want that. But if they don't need or want that, everything is simpler and they don't even need to think about channels.