MSC4179: Moderation event hiding

proposals/4179-moderation-hidden.md

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+## Motivation
+When an account with an offensive profile (profile picture, username...) is removed from a room,
+many clients will render the profile with a message such as "<offensive name> was banned".
+This essentially cements the string in the room history.
+
+## Proposal
+Add a top-level `m.moderation_hidden` key to the content of an event. The format is as follows:
+```
+{
+  "level": <level:string>,
+  "tags": <tags:[string]>
+}
+```
+
+`<level:string>` is a string referring to the *level* of percieved need to hide the event. Valid keys are:
+
+##### `"spoiler"`
+Show the event's presence, but hide its contents.
+
+##### `"hidden"`
+Hide it by default, display for moderators and client debug views.
+
+Tags is a list of strings representing content warnings that ought to be applied to the event.
+Clients can choose to match on the tag list when deciding how to interpret the event.
+
+Example:
+```json
+{
+    "level": "spoiler",
+    "tags": ["nsfw"]
+}
+```
+
+## Client implementation
+A client should offer a three-state option in its settings about the interpretation of the hints.
+- Respect the hints
+- Treat `hidden` as `spoiler`
+- Ignore the hints
+
+## Unstable prefix
+Use `org.itycodes.msc4179.moderation_hidden` in place of `m.moderation_hidden`.
+
+## Further work
+Standardizing the list of tags
+
+## Security implications
+None known