The link-preservation PR here already adds multipart-email regression tests. I'm the maker of InlinerAPI.

For teams editing notification HTML: could you try one independently expected action-link host/path or required instruction from a current change? Free, no signup; use synthetic values: https://inlinerapi.netlify.app/requirements

A reply with the actual result and setup/maintenance cost versus your existing checks would help establish whether another recurring check is worthwhile. No client files needed.

Standalone HTML checker, not a Plane plugin: it doesn't inspect the plain-text MIME part, verify permissions or test delivery. Thank you.