New MSC: A code for account suspension

Signed-off-by: David Teller <davidt@element.io>

proposals/MSCXXXX: A code for account suspension.md

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+# MSCXXXX: A code for account suspension
+
+This MSC introduces a new error code that servers may send to clarify that an account has been
+suspended *temporarily* but may still be reactivated.
+
+## Proposal
+
+### Introduction
+
+Matrix has a code `M_USER_DEACTIVATED` that a server may return to describe an account that has been
+deactivated. So far, this code has been used to represent accounts that have been *permanently*
+deactivated. In particular, clients that interpret this error code display it imply that the account
+has been *permanently* deactivated.
+
+However, some countries (e.g. UK) have laws that require the ability to appeal account
+deactivations. This requires the ability to specify that an account is *reversibly*
+suspended and let users know about the appeals procedure.
+
+This MSC simply introduces a new error code `M_USER_ACCOUNT_SUSPENDED` that servers may send to
+clarify that an account has been suspended but that the solution may still be resolved either by
+an appeal or by e.g. clearing up some abusive messages.
+
+This MSC does *not* specify a mechanism to suspend or unsuspend the account or to handle appeals.
+
+### Proposal
+
+Introduce a new error code `M_USER_ACCOUNT_SUSPENDED`. This error code MAY be sent by the server
+whenever a client attempts to use an API on behalf of a user whose account has been suspended.
+
+| Name | Type | Value |
+|------|------|-------|
+| `href` | string | (optional) If specified, a URL containing more information for the user, such as action needed. |
+
+The client is in charge of displaying an error message understandable by the user in case of `M_USER_ACCOUNT_SUSPENDED`,
+as well as a link to `href`.
+
+The web server serving `href` is in charge of localizing the message, using existing HTTP mechanisms,
+to adapt the page to the end user's locale.
+
+#### Examples
+
+Returning a static page:
+
+```json
+{
+  "errcode": "M_USER_ACCOUNT_SUSPENDED",
+  "error": "The user account has been suspended, see link for details",
+  "href": "https://example.org/help/my-account-is-suspended-what-can-i-do
+}
+```
+
+Returning a dynamic page customized for this specific user:
+
+```json
+{
+  "errcode": "M_USER_ACCOUNT_SUSPENDED",
+  "error": "The user account has been suspended, see link for details",
+  "href": "https://example.org/action-needed/please-redact-events?event-id=$event_1:example.org&event-id=$event_2:example.org
+}
+```
+
+
+### Potential issues
+
+See security considerations.
+
+### Alternatives
+
+We could reuse `M_USER_DEACTIVATED` and introduce an additional field:
+
+| Name | Type | Value |
+|------|------|-------|
+| `permanent` | boolean | (optional) If `false`, the account may still be reactivated. |
+
+in addition to the fields mentioned previously.
+
+### Security considerations
+
+This has the potential to expose private data.
+
+To avoid this, any `M_USER_ACCOUNT_SUSPENDED` MUST NOT be sent without authentication.