Should v3+ room version events containing an `event_id` field be rejected?

**Link to problem area**: https://spec.matrix.org/v1.12/rooms/v3/#event-format

**Issue**

We received [this pull request](https://github.com/element-hq/synapse/pull/17893/) to Synapse which ignores incoming room v3+ events over federation if they contain an `event_id` field. The rationale being that events in rooms with room version v3+ have their event ID [derived](https://spec.matrix.org/v1.12/rooms/v3/#event-ids) from the non-redactable contents of the event.

Given that the `event_id` field in such an event would thus be fed into that hashing algorithm, in order to generate itself, this event structure appears improbable. In addition, if such a field exists in the event, it's likely that in practice the sending homeserver is accidentally sending an event constructed for a room v1/v2 room, instead of the v3+ room it's intended to be placed in.

Should the spec clarify that an event received with an `event_id` field already included, destined for a v3+ room, should thus not be accepted by the receiving homeserver?

Start with the room v3 event format and event ID sections linked in the issue, then review Synapse pull request #17893 for the behavior under discussion. Determine whether the specification should require rejecting v3+ events containing event_id; done means the decision is resolved and the relevant normative text is clarified if needed.

Written by the indexing model from the issue text.