Which macOS tools require Python 2 (rather than Python 3)?
As well as xattr, other scripts in /usr/bin/ are smtpd.py and systriage.

A number of Automator actions for working with PDFs have at their heart python scripts -- the ones for combining PDFs, watermarking PDFs, extracting pages and converting images.

Quite a few scripts in Server.app are python.

It's not so much that they're definitively "not compatible" with python3, but that they've not been updated for over 10 years and haven't been tested. The scripts have not been updated to ensure that they will continue to work when called in other scripts and workflows.

Catalina will be the last version to include python 2.7, and it does so purely to ensure that these 'legacy' scripts will continue to work without any extra effort.