What the dickens is prev_state?

[DMRobertson]

Link to problem area: List of keys to be preserved by redaction in any of the room versions, e.g. https://spec.matrix.org/v1.2/rooms/v1/#redactions

Issue
The list of keys to be preserved by redactions mentions a prev_state key. I cannot find any other reference to this key in the entire specification.

dmr on titan in matrix-spec on  main via  v16.14.0 
2022-05-26 19:08:17 ✔  $ git log -n 1 --oneline
  bb47f08e (HEAD -> main, origin/main, origin/HEAD) Remove broken "token-based" link (#1081)

dmr on titan in matrix-spec on  main via  v16.14.0 
2022-05-26 19:08:19 ✔  $ rg prev_state 
content/rooms/fragments/v1-redactions.md
18:-   `prev_state`

content/rooms/v8.md
54:-   `prev_state`

content/rooms/fragments/v6-redactions.md
18:-   `prev_state`

content/rooms/v9.md
54:-   `prev_state`

Erik mentions it in #43 (comment), imported from Jira in late 2014. But otherwise... it seems to remain as a historical artefact?

[DMRobertson]

It seems that MSC2176 proposed removing prev_state from this list, amongst other things. It has been implemented but not yet assigned to a room version proper. In that MSC, Rich writes:

The following event keys are to be removed from the list of those to be preserved by a redaction:

membership
prev_state

(Note this refers to the event-level membership property, rather than the similarly-named sub-property under the content key.)

Rationale: neither of the above properties have defined meanings any more in the Matrix protocol, so there is no reason for them to be special-cased in this way.