Comment 34 for bug 1988819

Proposed fix in:

https://salsa.debian.org/apt-team/apt/-/merge_requests/327/diffs?commit_id=483e3b5c49762306c0a9f54117fd70cff43af4be

The corner case with kept back packages not due to phasing ends up with a notice before the prompt:

Reading package lists...
Building dependency tree...
Calculating upgrade...
The following NEW packages will be installed:
  phased-new
The following updates have been deferred due to phasing:
  phased phased-dep phased-depends-ready-dep
The following packages have been kept back:
  depends-phased-dep ready-dep
The following packages will be upgraded:
  depends-phased-new phased-security
2 upgraded, 1 newly installed, 0 to remove and 5 not upgraded.
Need to get 0 B/126 B of archives.
After this operation, 43.0 kB of additional disk space will be used.
N: Some packages may have been kept back due to phasing.
Do you want to continue? [Y/n]

There may be a nicer way to do this.