Should `unsigned` be a required field of events?

The spec has always considered it (and the fields which are always present) optional, but it's unclear if in practice it's treated as optional or as required.

This issue serves to untangle it: do pre-1.0 implementations treat `unsigned` as required/always present?

Start by reviewing the Matrix event specification and historical pre-1.0 implementations to determine whether unsigned is required or optional in practice. Document the evidence and update the relevant specification text once the behavior is established; the issue names no specific file or test.

Written by the indexing model from the issue text.