Based on pornels' example, "Semver-based package managers" are most modern ones (npm, bundler, cargo) that have the concept of Semver embedded in the version constraints for dependencies.
E.g. the tilde (~) and caret (^) operators in npm[0] allow you to specify version constraints on dependencies, that allow them to be resolved/pinned to higher minor/patch versions of that package, but not to a higher major version since that will by the Semver definition contain breaking changes and those might impact your project.
If that's true then Rich Hickey has refuted this approach to my mind. See my link in OP. I also mentioned npm and bundler in OP. This is precisely the approach I'm against. It's good locally, but deteriorates the eco-system at large when everyone follows it.
I'm curious what you think of the two posts I was basing mine on:
"Spec-ulation" by Rich Hickey: https://www.youtube.com/watch?v=oyLBGkS5ICk
"Volatile Software" by Steve Losh: http://stevelosh.com/blog/2012/04/volatile-software
I think there may partly be a "universes colliding" effect here, and partly just the future being non-uniformly distributed.