Based on EOL Timelines of CentOS8  and OpenStack releases there are slightly different plans for each release, you can find plans with respect to different RDO releases wrt CentOS8 and CentOS8-stream at [1][2]. The plans mentioned in the links are already in place.

Once CentOS9-stream is out can expect future releases on that, recent discussion on that wrt plan can be found at [3].

[1] https://lists.rdoproject.org/pipermail/users/2021-January/000967.html

[2] https://blogs.rdoproject.org/2021/01/rdo-plans-to-move-to-centos-stream/

[3] https://etherpad.opendev.org/p/centos-stream-9-upstream