nbdkit 1.40 was released on 22nd July 2024:

https://libguestfs.org/nbdkit-release-notes-1.40.1.html

We should rebase RHEL 10 to this version before beta.

This version is backwards compatible so there shouldn't be any regressions or changes in behaviour. There are various new features and some (low importance) security fixes.

The current version in RHEL 10 beta is 1.38.0.