This has been available for a long time. I believe the paid plans are aimed at supporting documentation for private repositories, and private enterprise codebases. It's part of the long-term sustainability plan for supporting high-quality documentation for open projects.

The pricing page dates all the way back to at least 2015: https://web.archive.org/web/20150706045846/https://readthedo...

The archive version states that it's free for open projects, and that statement is still there today. I think most people just end up on rtd pages for specific projects, and never see the rtd home page (or pricing page).

I agree, this doesn't look great, but there is still a free tier: https://about.readthedocs.com/pricing/#/community

...and the real question is, for how long will it remain available?

Since the paid tiers have paid for the free tier for at least 9 years, I'd imagine that it shall remain available for at least a few more years.

This is one case where something distributed would be of real benefit.