From DMcG:
For those who have not heard about it, worst case planning document was leaked to The Sun. Although other news sources have reported on this, it seems appropriate to link to The Sun's article.
Yes, it is a worst case planning document. But some of the key risks- no trade deal and a second wave, for example - are not unlikely.

Immediately followed by Dave the Gnome:
I wonder why these worse cases were not explained in 2016.

Possibly because they weren't understood at the time, particularly the risk of a second wave of Coronavirus when we hadn't had a first wave. To what extent may that second wave exacerbate any possible problems at borders, how could that have been foretold?