My understanding is that it exists already in the spec (and I think Synapse might even support it??). It is just that no clients have implemented it yet... IIR

My understanding is that it exists already in the spec (and I think Synapse might even support it??). It is just that no clients have implemented it yet... IIRC, the Element team has been working on it, but they seem very hesitant to release anything around threads that is half-baked. They really want to get them right the first time!