I find it very difficult. I would be a bit anxious merging the code this close to the final release, so I imagine the release managers would be much more.

Anyway, this is the current state of this issue from my perspective:

As far as I understand, progress is currently blocked because Matthias thinks this mechanism must allow Debian to replace all related downstream patches.
I disagree with him in the way Debian is patching Python, which IMO is way too intrusive and fragile. Given that this vendor config will essentially shift the burden of supporting these modifications to the Python upstream, I don't think it would be a good idea to allow the modifications Debian wants in this config.
For all other downstreams I have talked to, the proposal seems to be fine, though more testing would definitely be appreciated.

I think that at this point, we could really use a core dev to help push this to the state of being able to get merged, because I don't think I can do that alone.
I have already requested feedback from the community and the technical discussions on the PR went silent, so I do not see what else I can do :/
If needed, I would be available to brief anyone on my understanding of the issues and how this proposal handles that. So, any core dev, please feel free to reach out if you want to help move this forward.