> Presumably the ones where you are expected to have the latest version of everything and make a new package if you break that (python2 → python3)

This is essentially Go's MVS. (Can only specify a minimum bound, can't depend across major version bumps). Any others? MVS is more the exception than the rule.

> Not sure why more ecosystems don't do this. Sure an update could break dependents, but, like, you already have a big problem if a dependent was keeping you on an old version no matter what.

I'll bite :-) You lose a lot of expressivity. Incompatibility is specified from the dependee, despite being a property of the dependency. I can point to instances where this has causes issues; e.g. dependees using unstable APIs. That's exactly why MVS uses the _minimum_ bound, to minimise such breaks.

> Building a SAT solver into the package manager seems to be a solution in search of a problem.

I agree in that there are better algorithms for error reporting! But NP-hardness is a pretty fundamental property of dependency resolution and removing it moves the pain somewhere else.