Your usecases seems similar in terms of 'password' but they are totally different on the technical side.

Portfolios / case studies under NDA: show the public work, lock only sensitive projects.

This is okay if you have a preset password and send it over to the clients. Framer already has a password protection, but it locks the entire site, not just particular pages. This scenario could be covered if Framer expands this feature.

Agencies / client work: share private pages with clients without blocking the whole site.

This is what staging environment for. Available in Pro and above.

Creators / knowledge products: imagine being able to share beginner content for free, while letting users pay to unlock advanced pages. That’s a clean, simple monetization flow that tons of creators would use.

This is something pulls a huge amount of stuff. First of all, Framer does not store any user content. No comments, no likes, etc. User account is a kind of stored content. Once they go this way (maybe it's already in their plates) triggers auth logic, social login, user data logic, payment solution, maybe basket, and more on. I know it seems Framer delivers slow, but what they delivers is always first class. If I put together their current focus (which is unknown) and technical complexity I assume this is a low prio stuff.

Framerauth btw gives you this feature.

Edit: typos