ChatGPT now offers "project-only memory" - when selected, chats in a project can reference previous conversations from within that project, but not any conversations from outside that project.

To try it, create a new project and click "More options" in the lower left (which then becomes "Memory" and lets you pick Default or Project only).

I tried editing an existing project but there was no way to select project-only memory - literally the only option was editing the name.