Yes, this is a breaking change, it will break the code that reads gzipped temporary files.

Note also that even if this change be accepted, you could not use this feature until Python 3.9 be released. But you can write to temporary files in all Python versions if specify an explicit mode.

So the benefit is small, but there is a risk of breaking existing code.

There is an opposite proposition: deprecate guessing the GzipFile mode (issue28286).