I do 99% of the work, but of course I also accept contributions. However, PRs are very rare and are mostly about bugfixes, not new features. Bugfixes are always released as part of the non-Insiders version, of course.

What's probably interesting is to consider the user demographics, about which I learned more in a survey [1] I did last year. Only 8% of my user base would consider themselves Frontend engineers. Given that this is an HTML template with some rather complex CSS/JS, it's not surprising that there are only very few PRs that implement new features. I guess most Frontend engineers go for JavaScript-based solutions like Vuepress or Docusaurus. Material for MkDocs is Python-based.

[1]: https://twitter.com/squidfunk/status/1259832077668810752