The EU was never going to entertain any such protocol for Scotland, and if Yes advocates ever thought it might, they were deluding themselves. The reason the EU negotiated the NI Protocol was Ireland’s stated vital national interest, promoted in a diplomatic tour de force. https://twitter.com/unisondave/status/1381132531563692034

The crucial thing to understand here is that from an EU26 perspective, the Protocol is an act of solidarity towards Dublin - not Belfast. Once you grasp that, it becomes obvious that the prospects of a special deal for the Anglo-Scottish Border are non-existent.

This seems to have provoked ... a fair bit of discussion. In the interest of fairness: I’m referring to an economic border here. My guess is that an opt-out from the Schengen Area to protect the CTA would be quite plausible, not least because Ireland would probably be supportive.