[morvita]

Python 3.5 was declared end of life in September 2020, so it makes sense for library developers to drop support for it as well. Something like Flask 2.0 may work in Python 3.5 (I haven't tested it myself), but it isn't worth the developers' time to guarantee that support for a language version getting no support upstream.

https://www.python.org/downloads/release/python-3510/

[ehsankia]

Deprecation aside, Flask 2.0 begins adding type annotations, which is 3.6+

https://github.com/pallets/flask/pull/3973

[matthiasv]

Type annotations started with 3.5, 3.6 added type annotations for variables.