Short answer: without a policy change, not until Melodic goes end-of-support.
The snapshot repositories currently serve two functions:
Supplying "final" archival snapshots of unsupported rosdistros, initially motivated by the build farm compromise.
Providing snapshots of each ROS (1) LTS and ROS 2 sync on their primary platform, in all cases so far the Ubuntu LTS release that they target.
In the first case we tried to archive everything since we were going to be removing the packages from the primary ROS repositories. For the regular snapshots we started with just the Ubuntu LTS release that each LTS rosdistro targets and the recurring sync snapshots scripts actually have the assumption of one rosdistro :: one distro baked into the current snapshot generation scripts. I've dodged the issue for the final snapshots by doing them all by hand which has been fine since you only do a final snapshot once but it'd be too much for every new sync.
The snapshot repositories are still in "beta" state. They've become somewhat solidified by the fact that we're using some of the same infrastructure for the package archive but that was a decision made in a time sensitive situation rather than one where we had the luxury of looking out at the horizon.