Reason: I don't see such tags on the majority of the releases
Based on the latest doc:
v[Major].[Minor].99 - A tag applied to master branch to signify that work on v[Major].[Minor+1] has started. For example, v1.7.99 will be tagged at the start of v1.8 process. The tag corresponds to VERSION_MAJOR/VERSION_MINOR/PATCHLEVEL macros as defined for a work-in-progress master version. Presence of this tag allows generation of sensible output for “git describe” on master, as typically used for automated builds and CI tools.


Here are the only .99 tags I find in the repo