Probably bait and switch. They call both ChatGPT, so now people believe they will get the better old ChatGPT, but they get the new cheap and worse ChatGPT "Turbo" that they switched to recently. Fewer will realize if they no longer give you the option to use the legacy version in this API.

They did not release the older more performant model to the API. Please ask them to on the Discord or Twitter. But I think they will not. There is too much demand to handle and the older "less streamlined" models are very problematic for them (based on the fairly constant API/ChatGPT problems and well known incredible demand).

I get the impression that until there is a significant amount of excess capacity, they will not put out new larger/slower models, so the only way you get a better one is if they can still make the next ChatGPT model release just as fast/"lightweight".

My suggestion is to find specific abilities that seem to be lacking in Turbo, and try to get a message to OpenAI staff about it with a request to attempt to improve the next ChatGPT model in that way.

Having said all of that, text-davinci-003 is still available.