It's a bit of both. The GPT-4 models have definitely been changing - there's multiple versions right now and you can try them out in the Playground. One of the biggest differences is that the latest model patches all of the GPT-4 jailbreak prompts; quite a big change if you were doing anything remotely spicy. But OA also says that it hasn't been changing the underlying model beyond that (that's probably the tweet you're thinking of), while people are still reporting big degradations in the ChatGPT interface, and those may be mistakes or changes in the rest of the infrastructure.