I think that if we implement this feature behind a flag then some libraries will start requiring that flag to be set.  Which will inevitably lead us to a situation where it's impossible to use asyncio without the flag.  Therefore I suppose we should either just implement this behaviour by default or defer this to 3.9 or later.

I myself am -1 on making 'run_until_complete()' reentrant.  The separation of async/await code and blocking code is painful enough to some people, introducing another "hybrid" mode will ultimately do more damage than good.  E.g. it's hard to reason about this even for me: I simply don't know if I can make uvloop (or asyncio) fully reentrant.

In case of Jupyter I don't think it's a good idea for them to advertise nest_asyncio.  IMHO the right approach would be to encourage library developers to expose async/await APIs and teach Jupyter users to "await" on async code directly.  

The linked Jupyter issue (https://github.com/jupyter/notebook/issues/3397) is a good example: someone tries to call "asyncio.get_event_loop().run_until_complete(foo())" and the call fails.  Instead of recommending to use "nest_asyncio", Jupyter REPL could simply catch the error and suggest the user to await "foo()".  We can make that slightly easier by changing the exception type from RuntimeError to NestedAsyncioLoopError.  In other words, in the Jupyters case, I think it's a UI/UX problem, not an asyncio problem.

Yury