In Bullseye release file:/usr/bin/python is not reserved, but intentionally unused. In Bullseye release neither deb:python2 nor deb:python3 packages own /usr/bin/python. This is a Bullseye Release Goal with consensus from all cpythons/pypys/etc interpreter maintainers, modules maintainers, and app maintainers. Specifically cpython2.7 is still around to allow for re-bootstrapping of pypy* ecosystem on existing or new architectures. No packages in Bullseye may depend, or build-depend on neither deb:python, nor deb:python-is-python* packages. Thus it is a leaf package without any dependencies. By definition not impacting any unrelated software. It is intended for this package, and all of its binaries to be removed in some future Debian release after the next one. I hope you notice the irony in the package name src:what-is-python, it is meant for local admins to deploy as compatibility with their respective venvs and other estates only. It is intended for nobody to use "/usr/bin/python" outside of venv for now. And for them to change their scripts to use either python3 or python2 explicitly.
I want to say this bug report is "bullseye-ignore" or "wontfix".