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.