If a fix for this can't be released on time, I'm requesting an exception for
llvm-15. Removing OpenVDB from Debian will affect Blender, which is is
relatively high-profile and should not just be axed for the sake if a pruning
operation.
You still have time, we aren't going to release the trixie anytime soon :) but
we won't probably block the removal in testing for openvdb (the popcon isn't
high IMHO).
Let's hope upstream notices the issue and fixes it.
In the meantime, it may be possible to remove the immediate pressure by simply
disabling the build of libopenvdb-ax. The rest of OpenVDB doesn't require LLVM
15, and I couldn't find any Debian package that depends on it (save for
python3-openvdb, which will simply not have AX support).
This should at least bring Blender back into trixie.
I did a quick build test with the AX component disabled, and that seems to work
fine. Blender also compiles. Didn't try to install and run the final result,
though.