Drupal core minor releases sometimes break a small number of contrib modules, that then need a new, compatible release. Generally this happens quickly enough but there can be 2-4 weeks where people update to the minor release and have problems.
Specifying an upper bound of compatibility with core has problems in the other direction because then you need a new release every six months even if nothing changed, for what is more likely to be a once in five years or never event for each module.
From @lauriii, add an option to override conflicts in the packagist endpoint, this could be in the project UI itself - it would need to apply for all releases, not just the latest one. This would allow us to retrospectively mark releases as incompatible with core, then the new release could remove the flag again.
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An additional scenario that this would help with is when people upgrade contrib without updating all modules. At least with this, they would get a warning that the module is incompatible with the core release. This could be happening in cases where modules have been pinned to a specific release for example due to a patch that applies on top of the specific version.