What about #14589 (the towncrier PR)? It looks like the necessary upstream PR (twisted/towncrier#157) already went in.
Also, given that I'm not the one doing the merging but probably one of those creating the most work there, I'm happy with whatever solution you prefer, including towncrier, even if this requires some changes on my workflow...

@dstanby I think that your proposed workflow would work fine.
Out of curiosity, how much changes do we have per releases and how are they distributed between the four categories you are proposing?

In terms of writing style, that should be fixed prior to merging the PR. Might be worth adding a note to the reviewing guidelines about that.

@anntzer even though it's merged, it's not released and I don't think towncrier is the most actively developed project.

I think (w/ no real evidence) adding to the same file that already has some changes will help first-timers see what kind of thing to write, and will hopefully reduce the amount of copy editing that needs to be done at release time.

@NelleV Most stuff by quantity is deprecations, but excluding @anntzer's commits(*) I'd say it's fairly well split between the 4 categories.

(*) Not a complaint at all, but he seems to do most of the deprecating ;)

I know this will involve merge conflicts along the way,

We used to do it that way and it caused so many re-bases that we moved to the many files.

Could we do a mixture of both? Split between four categories, but keep several files with a strong naming conventions and structure, and "simply" do a cat before the release?

We discussed this on the weekly call, and there was support for exactly the approach in this PR. Yes, there will be some rebasing needed, but usually new developers who would be burdened by rebasing are not doing deprecations etc which get hit heavily. And splitting into four categories lessens the chance of conflicts.

Woops, not sure why I closed this!

@dstansby What is the state of this? I think we want to get this enabled for 3.3 ASAP!

Thanks for the nudge, I think modulo getting the docs to build this is almost there. Will try and sort this out in the next couple of days!

I think this is ready now!

Doc builds complain:

/home/circleci/project/doc/api/api_changes.rst:40: WARNING: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: 'api/api_changes/api_changes_3.2.0.rst'.

There's four API change notes on master so far. I can handle these in one go.

Open PRs will need an update by the respective author.