Broadly there are two possible reasons why ChatGPT could have degraded (not saying it has).
1) OpenAI have higher user base than expected, so costs are very high/compute is limited to serve the full model, so they are using speculative decoding with a changed threshold to reduce costs.
2) OpenAI have changed the weights or control structure of the model in a way that is negative for performance. There is are two possible motivations a) reducing hallucination and/or ‘unwoke’ responses that might embarrass msft/openai or b) reduce capability of free/consumer product to push their new enterprise stuff.
Theses are clear motivations for openai to make the model worse than it was. There is no conclusive/rigorous evidence either way, and anecdote seems to lean towards it being nerfed.