For me, minutes ago, as a Plus subscriber:

I started up Codex CLI fresh. That version of Codex was 1.42.5. 5.6 wasn't in the models list.

After I updated Codex to a newer version (0.144.0), 5.6-terra and -luna appeared in the models list (but not 5.6-sol).

(It's impossible for me to know whether updating was causative or just correlative, but that's the timeline I experienced.)