It looks like we will replace Netlify with Read The Docs for pull request preview of all documentation. See Create preview build · Issue #4 · plone/plone-sphinx-theme · GitHub for discussion and Update readme by stevepiercy · Pull Request #8 · plone/plone-sphinx-theme · GitHub for proof of concept. The only downside is that RTD publishes the comment to the pull request upon initialization of the build, and does not update it when the build completes or fails, like Netlify. However this is the only viable replacement I found. Feedback appreciated.