I just stumbled upon: https://docs.fedoraproject.org/en-US/packaging-guidelines/deprecating-packages/#_marking_a_package_deprecated I am not sure if this is used at all. But we can add something like: Obsoletes: deprecated() < YYYYMMDD
Deprecation and having this package obsolete things aren't really linked in any meaningful way. For one, not all packages which are marked as deprecated would need to be obsoleted by this package. fedora-obsolete-packages is to be used only in the case that something has to be removed from existing systems to prevent dependency problems. That's all. Some kind of blanket obsolete of anything marked as deprecated would not be warranted even if it worked in the way you suggest.
A deprecation is a promise. We promise that this will go away some time. Than we retire it (or not). Than we might need to obsolete it.