The OpenVDB developers have merged a fix for the LLVM 18 compatibility issue:
https://github.com/AcademySoftwareFoundation/openvdb/pull/2065

This fix is not in any release version yet, but once available, please revert
the patch that disables AX (since Debian version 10.0.1-2.3) before pushing a
new release.
See bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1061216 for more
information.

Thank you!