Probably some combination of quantizing down from original fp16 weights and changes to the system prompt used for chat. Both can cause degraded quality, the former more than the latter.
Re:GPT-4 API General Availability
by PeterStuer on Friday July 07, 2023 @02:46AM
(#36628090)
"ChatGPT isn't even able to solve simple Python problems anymore, even if you try to help it. And some time ago it did these same problems with ease."
This is my experience also. I have not formally benchmarked the different releases, but specifically for Phyton coding ChatGPT 4 got considerably worse with the latest updates.
My guess is that they began to restrict ChatGPT because they can't sell that. They probably want to sell you CodeGPT or other products in the future so why would they give that away for free? ChatGPT is just a teaser.