Thanks for letting us know, janust. I confirmed that `asyncio.TimeoutError` no longer works for the code example in 3.8 and that replacing it with `concurrent.futures.TimeoutError` works correctly.

Before moving forward with a PR to the docs, I think we should wait for feedback from Yury to check if this behavior is intentional for `run_coroutine_threadsafe()`, I'll add him to the nosy list.

To me, this seems more like a side effect of `asyncio.TimeoutError` being changed to BaseException rather than an intended consequence. The main purpose of that change was to ensure that TimeoutError wasn't unintentionally suppressed by generic try-except blocks. IMO, it seems rather counter-intuitive to have to specify `concurrent.futures.TimeoutError` when using a timeout for the future returned by `run_coroutine_threadsafe()`. I'm not certain that it can be avoided though, other than by changing it to return an instance of asyncio.Future instead of concurrent.futures.Future.