Here's a tracking issue for things to do before removing the legacy single substream:

• (1) Implement answering finality proof requests. (Add a protocol that answers finality proofs #5718)
• (2) Switch to the new blocks protocol. There should probably be a CLI flag that makes it possible to disable this, in case things don't work. (Use new block requests protocol #5760)
• (1) Send the Role in the registered notifications protocols (Pass an encoded Roles as the notifications protocols handshakes #5665)
• (1) Send the Status message in the handshake of the block-announces protocol (Send a status message on block announce handshake #5726).
• (2) No longer make it mandatory to open the legacy substream. Can only be done after a release that includes all the items with (1).
• Remove the legacy substream. Can be only be done after a release that includes (2).