When we can say we support all computers to run without UIThread ?
We'll never be able to say that, as computers are provided via extension point and has a lot of extensions in the wild that have worked for a long time, sometimes/often assuming they're running in UI Thread.
I mean at some point we need to drop the UIThread support and only support Async completions.
This would be quite similar to breaking an API and cause a major risk of breaking many existing clients.
If we're to change that, we need a clear plan to communicate about it, plan the change, handle UI Thread requests in a way that reports a more helpful message... That's actually a lot of work, while we already have a way for extensions to declare whether they require UI Thread and already have a way to see which computers do prevent async completion from working; I'm not sure breaking compatibility with UI Thread based computers would bring much more.