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).