The announcement says they're rolling it out over the next 24 hours or so. I think it's reasonable to do a slow-roll-out release for one of the most used products on the internet.

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.)

GPT-5.6 Terra just showed up in Codex for me.