It is used to check the meminfo-writer SELinux label, but as you noticed, it’s not there anymore since it was moved.
The new /usr/bin path was added in version 4.3, but it was not backported to version 4.2, which is causing the error:
One of the Fedora 42 changes was unifying /usr/sbin and /usr/bin directories (more information here). Some of the Qubes OS utilities had to be relocated from the former directory to the latter.
Look into /usr/bin directory. The meminfo-writer should have been relocated there.
The patches goes through thorough testing. It might take weeks or even months until they are delivered to stable branch.
Unless you’re having issues with SELinux labels, consider this line a harmless warning.
You can still fix this manually by editing /usr/lib/python3.11/site-packages/vmupdate/agent/source/plugins/fix_meminfo_writer_label.py in dom0 and by replacing /usr/sbin to /usr/bin in the meminfo_path variable.