Like https://mylittleforum.net/forum/? I am also looking for non-js + open source (self hosted) forums and I don't find much anymore.

PhpBB [0] was the king back in the days.

[0] https://www.phpbb.com/

Making such a frontend for remark42 with complete server-side rendering should be possible. In fact, even now the backend doing a lot of this, for example comments preview rendered this way.

Do you mean front end for mobile or desktop?

I meant server-side rendering for the web. Remark42 exposing all functionality via API and any alternative frontend can be made. I know someone doing android SDK for this. Not sure about ios side.