deprecate starting verifications without first requesting

proposals/xxxx-deprecate-starting-verifications-without-request.md

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+# MSCxxxx: Depretate starting key verifications without requesting first
+
+Currently, the spec allows a device to begin a verification via to-device
+messages by sending an `m.key.verification.start` event without first sending
+or receiving an `m.key.verification.request` message.  However, doing so does
+not provide a good user experience, and allowing this adds unnecessary
+complexity to implementations.
+
+We propose to deprecate allowing this behaviour.
+
+Note that verifications in DMs do not allow this behaviour.  Currently, Element
+Web is the only client known to do this.
+
+## Proposal
+
+The ability to begin a key verification by sending an
+`m.key.verification.start` event as a to-device event without a prior
+`m.key.verification.request` is deprecated.  New clients should not begin
+verifications in this way, but will still need to accept verifications begun in
+this way, until it is removed from the spec.
+
+## Potential issues
+
+None.
+
+## Alternatives
+
+We could do nothing and leave it in the spec.  But we should clean up cruft when
+possible.
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+No unstable prefix is removed since we are simply deprecating behaviour.