For the record, nobody complained when I removed all the class diagrams and other things mentioned above, so my takeaway was that ... yes, those features were useless :) On the other hand, people complained about lack of essential features that Doxygen didn't have, like proper search or #include information for non-class members.

Sure. You're doing useful work shaping something that is a bit too free-form for most. I didn't question your curation. Nor do I think your project needs to support them.

But the fact that you haven't heard from the inevitably short list of projects that have looked for new documentation tools in the past ~14 months and also happen to be complex enough to benefit from niche features like inheritance diagrams doesn't mean they don't exist.