(More-so if you have any apps of your own, or if you are using some from elsewhere - and they suddenly stop working, then maybe reach out to check the creator knows about these deprecations.)
It seems it's not just GPT's that are vanishing on Dec 11th.
API access to the following six models will end on December 11, 2026:
GPT-5
GPT-5 mini
GPT-5 nano
GPT-5 pro
o3
o3-pro
To avoid any disruption, they recommend migrating to the replacement models mentioned here
before the shutdown date.
If you have any questions about model migration, You can reach out on the OpenAI Developer Forum here:
Hope that helps