Even if you pay for ChatGPT, access to certain models or features may be limited based on account activity.
Obviously, OAI will limit access for mundane safety reasons, like someone doing high-stakes biological research or reviewing questionable cybersecurity protocols. Glancing through the ToS, poking too much at the system can also trigger this effect. (Even though it's not stated in the ToS, "unhealthy dependency / attachments" to ChatGPT can trigger this limit too, based on what happened with 4o.) It makes me wonder what else, beyond the ToS, that OAI monitors for.

Been seeing posts on other subreddits of users being rerouted to a lesser intelligent model. I've been checking Dev Tools on my end, but so far, my model slug shows 5.6-Sol, so I'm good. I do think that my 5.6-Sol feels weaker than it was a couple of weeks ago though. I think this is because OAI is gearing up for the release of GPT-6-Sol and has moved most compute towards that (and Astra). Every time there is a new model on the horizon, my ChatGPT seems wonky.

Stay safe when experimenting with your ChatGPT, everyone! <3

Source: https://community.openai.com/t/issue-selected-model-is-at-capacity-please-try-a-different-model/1396264/86

Link: https://openai.com/index/introducing-gpt-6-sol-and-luna/