There are no releases planned targetting Ubuntu Core specifically afaik.
REP 2000: ROS 2 Releases and Target Platforms documents current releases and the platforms supported by those. Note that supported can mean different things: binary packages for a package manager or from-source builds on-target (or via cross-compilation).
The main difference between Ubuntu Core and "normal" Ubuntu releases would be the use of snaps for packaging software (and updating, etc). There are currently no dedicated builds in place to generate snaps.
Edit: my personal understanding of snaps and Ubuntu Core would also suggest the highly federated nature of ROS does not necessarily translate 1-to-1 to distribution via snaps. Or at least, not as per-package snaps. It's apparently possible to build snaps out of ROS packages (see Robotics Recap: Learning, Programming & Snapping ROS 2 for instance), but it's typically done for entire applications or larger parts of them, not for individual packages (that would be possible though). Whether snaps-per-package scales, I don't really understand at this point.