I had never heard of this service but indeed, it seems to be a registrar builtin to GCP. I suppose this is not going to be killed, and presumably the reason they aren't migrating domains users to it is awkward mapping to GCP projects? It would be great to at least have a button to copy it over for people that do have a GCP project for it...
Edit: it looks like there may indeed be such a button / command. Really strange they wouldn't communicate the option to keep customers on their services!