[Feature request] introduce an upgradability marker for packages that constrain updating of other packages

[rapus95]

This is the complement to the yellow marker (which says “there's a new version, but we cannot use it due to dependencies”) and shall mean “this package prevents us from updating dependencies”. I'd propose a warm color for it. Maybe even red. It helps by directly showing which packages need a new compat info (open PR/issue on their repo) or need to be disabled if I need the new version of another package.

[IanButterworth]

This already exists. It's a bug in #3150 that it's not showing

[IanButterworth]

Sorry, I misread this suggestion. I'll reopen for discussion.

My view though is that there isn't much more the proposed marker would add, and I think adding another marker type might get close to making the information too complex.

If there's a package being held back, one can use --outdated to find out why. That singular example might benefit slightly from the suggested marker, but often there are many packages being held back in various ways, so I don't think a single new marker type can communicate useful information. For that the user needs to use --outdated