I added "Remove Trailer from existing header fields" for the dechunking algorithm in RFC7230, and at some point we added Trailers to the list of fields to be removed hop-by-hop even when not present in Connection. However, now that preserving trailer fields is optional and separate from header fields, it makes more sense to retain Trailers as a sort of tombstone that indicates "something might be in the trailers" or "something might have been in the trailers but we lost it along the way".