Context from https://groups.google.com/g/jenkinsci-dev/c/kQ-LLGpETQs/m/YCgrcojWAgAJ

… updating to a newer Jenkins results in bundled plugins being updated to the bundled releases if what's installed is older … downgrade would need to happen on every bundled version bump though, which given the regularity of plugin releases would be practically every LTS bump (not to mention the lack of support for downgrading configuration). It used to be possible for users to "pin" versions of plugins, but that was dropped in or around 2.0 when we no longer installed bundled plugins by default, making it basically an offline fallback only (resulting in the current policy IIRC).

I propose this behavior be changed as follows:

split-plugins.txt should be enhanced to also track the minimum version to be installed (possibly declaring that, if undefined, is equal to the split version, so no change to the file needed). Only if the installed version is older than that one should the bundled version be installed.

This would stop automatically updating installed plugins on core update just because they're bundled, while allowing core to declare an update to be necessary and to enforce it on update (while still allowing downgrade).