the fix for PR ada/117996 causes a change for the gnat-13-<hash>
provides, causing uninstallability for all ada packages.  This fix
doesn't look like an ABI change, neverless it triggers one on the
packaging side.

If I understand it correctly, we still plan to switch to gnat 14 for the
upcoming trixie release, but this will be a problem for backports, and
also assuming we had the switch to gnat 14 already done, we would have
the problem there again.  Is that hash too strict, or is this a real issue?

according to the tracker, https://tracker.debian.org/pkg/gcc-13
there are no abi changes on ppc64el. why?

lowering severity, that commit has been reverted for now.

No idea.  I would also not expect these changes to break the ABI.

The language requires that all reverse dependencies are rebuilt after
any change in the Ada sources of a library, so the hash is correct.

Until gnat-12, a change in libgnat was unnoticed until someone
attempts to rebuild an Ada package, and the fix was to rename all Ada
-dev packages.

With the hash/Provides mechanism, such a change is immediately visible
as autopkgtest failures, and can be fixed with a bin-NMU of all Ada
libraries in proper order (this remains an option).

However, I see that you have reverted the fix for PR ada/117996 from
libgnat-13, and it make sense to only apply it to gcc-13 after the
gnat-14 transition..  It will then affect Ada libraries neither in
bookworm backports (gnat-12) nor in trixie (gnat-14).

From its description, I suspect that PR ada/117996 does not affect
many users in testing/unstable.