Development of new features (and hopefully introducing new bugs) in kicad-nightly V5.99 is pretty much frozen, and for already quite some time all effort is going into making it ready for release. There is hope of KiCad V6-rc1 being released by the end of April, and an official KiCad V6 somewhere between a few months or half a year later.
Response to bug reports is excellent. I’ve reported 10+ bugs, and if it’s easy to fix, it’s usually done within a day. record is 34minutes between report on gitlab and a comited to git. More complicated stuff takes longer.
As far as I know there is no manual for the rules based DRC yet. There is a lot of info in this long thread: