Note: this is my interpretation and doesn’t necessarily reflect the development team’s intentions accurately, so take this with grain of salt.
Until very recently many issues in gitlab had the milestone for v6 even though it wasn’t certain if someone would implement them. Now there’s milestone 7.0 and some issues were moved to that: https://gitlab.com/kicad/code/kicad/-/issues?milestone_title=7.0.
There are now three feature development milestones: 6.0.0-rc1 , 6.0 Feature Freeze and 7.0.
6.0 Feature Freeze is for new (major?) features. For each proposed feature it must be decided if it actually will be implemented for 6.0, and now it seems to be the time to make those decisions. If nobody hasn’t volunteered to implement a feature, it won’t be in a condition to be included when the feature freeze moment comes, and it can’t be set to that milestone.
The rc milestone is mainly for bugfixes and other smaller changes. They must of course be ready when 6.0 is finally released.
I see 7.0 as preliminary in nature; ATM it includes only old feature plans which were postponed because of lack of manpower. They can be postponed even further, or maybe there will be 6.1 where something can be implemented.
6.0 Feature Freeze and 7.0 should correspond roughly with KiCad 6.0 Roadmap and KiCad Future Versions Roadmap but I haven’t checked how up to date those are right now.