This the deal Johnson was celebrated for agreeing. The piece of paper he waived in the air as victory during the December snap election campaign.
The deal requiring a customs border between Northern Ireland and rest of the UK was not an "unforeseen consequence". It was a consequence foreseen by May, which is why she wouldn't agree to it
The cave-in on this by Johnson was what got him a deal where May didn't
It was what necessitated an election to get rid of the government's reliance on the DUP, who could never support severing NI from GB.
And such recent history at that. This was all just 9 months ago!