There used to be a “Custom GPT” feature which basically just creates a prompt wrapper with some extra functionality like being able to call web APIs for more data. Can’t seem to find that menu right now, but it would have easily replicated the study feature. Maybe it was limited to paid accounts only.

Yeah custom gpts are only for paid users. However u can create a new project under "Projects", name it, then when u create it, you can see on the top right the three dots button, click it, open project settings, and there u can place your system prompt under instructions. Every chat you start in that project would send those instructions as a system prompt to the model you are chatting with. so essentially "Study Mode" could be recreated with this approach, or at least it should.

It’s still there, but the builder is only in the web UI.