See zulip/zulip#14711 and the corresponding issue thread for details, but basically:

• We now have a user_id field alongside the inconsistent user object in both the events and GET /messages code paths.
• The zulipchat.com/api/get-messages documentation now has basic coverage of the reaction element format and the changes here, though we don't bother discussing the non-user-id fields of the Legacy user dicts, as clients shouldn't be using them anyway.