gnome-chemistry-utils requires Mesa's libosmesa, which was removed in
2025 (Mesa 25.1.0) and is no longer available. The most recent version
of Mesa that still built libosmesa used llvm-toolchain-19, so this is
one of the factors keeping llvm-toolchain-19 in unstable.

The most recent maintainer upload of gnome-chemistry-utils was in 2021
and there have been three NMUs since then. The most recent upstream
release was in 2018.

Its only reverse-dependency appears to be gfsview, for which I requested
removal in #1147651 - if the ftp team goes ahead with removal, then that
one will need to be processed first.

As with gfsview, if these packages are still desired in Debian, I think
the only way to provide them would be to introduce the equivalent of
Ubuntu's src:mesa-compat (a separate source package containing an
outdated version of Mesa that only builds the libraries that were
subsequently removed upstream), after which these packages could be
re-introduced; but in the absence of someone introducing and maintaining
that package, I think it's time to remove these.

(This was prompted by packages that directly or indirectly depend on
llvm-toolchain-19 being proposed for autoremoval from testing.)

    smcv