By reading many of the comments here, it looks like that you are missing the point (maybe you are not a Flask user): it would be great if Flask would only introduce breaking changes in major releases. Unfortunately, many things break with minor releases too.
We develop a framework built with Flask, and it is very painful. We always pin Flask< minor version (not major).
This is unfortunately happening with other software too. The community should really align and stick to SemVer.