This should also go in the release guide updates?

Yes, probably; cc @ksunden

Is there any reason not to pin this way on main? Its our own first party dependency that we control the release cycle of. While we currently version it for every (feature) release, it could in theory slow down that it doesn't need to be updated for each mpl release.

I feel like updating the pin is already a part of the theme release (we only updated it because we want to change/fix something, essentially), but changing it to ~= from >= doesn't change that procedure, and we pretty much always want to pin it to the most recent version.

If it is pinned on main, then there is nothing special to do for release, and just the repinning to newer versions doesn't get backported.

fair point. I am in general against hard pins, but this is in an internal requirements file we use for setting up CI + developers machines (not something carved into stone on pypi or a packaging system) so I guess that would be OK.