I suppose it depends on your definition of "privacy-respecting" and "server-side" but, for example, Proton offer SIEVE filters you can configure with a web UI.

I suppose it depends on your definition of "privacy-respecting" and "server-side" but, for example, Proton offer SIEVE filters you can configure with a web UI. https://proton.me/support/sieve-advanced-custom-filters https://proton.me/support/sieve-advanced-custom-filters