Many projects are dropping support for old Python versions very fast. It's not surprising given the history of the Python 2 -> 3 transition. No one wants to end up in that situation anymore. For example, NumPy has already dropped 3.6 support (3.6 will be EOL later this year).
I don't know about "very fast", Python 3.6 was released in 2016 giving people plenty of time to update from 3.5. Those are backward-compatible too.
If you haven't updated in that time, you probably have a systemic problem which shouldn't be blamed on Python moving too fast (in fact I suspect those people don't keep up with bugfixes either, so nudging them to upgrade when they try to update flask/numpy is a good idea).
I’m guessing they want to tell people it’s time to upgrade. And 3.6 brings a lot of niceties that they might want to use, like f-strings.