I'll share a draft PR soon (excluding Decimal), so far it's still looking straightforward.

> Maybe old versions would correctly ignore the new bit being set.

That's one of the benefits of using bit flags in an ABI: backward-compatible extensibility.  (An implementation can defeat it by intentionally failing if unknown bits are encountered, but the code in question doesn't appear to be doing this.)

> You can round explicitly before formatting
>
>    >>> '%5.1f' % (round(-.00001, 1) + 0.0)

Yes, I have experience with it.

  1. even as a one-off, it's questionable.  If someone accidentally changes the precision in only one of the spec string or round call, that's a bug.
  2. since applications and libraries may pass around format specs, and because of (1), you'll try to make a programmatic solution.  Now you're parsing format spec strings.
  3. while a programmatic solution can be done for a function API like format() that takes a separate spec and value, there is no sane way to wrap f-strings