> This can make harder writing portable code that works in 2.7, 3.4 and 3.7.

asyncio for Python 3.4 is fairly outdated.  Most of the async packages today require 3.5+, as they usually use async/await syntax.  I say this sort of backwards compatibility (showing a warning) isn't really a big concern.  A bigger concern for us is new code using 'with await lock' pattern, hence the warning.