Bret Devereaux

'when Rome fell humanity became far less literate overall, people lost the ability to read and write, they lost the ability to draw properly, they lost the ability to build in the way the Romans did...Things can go backwards' Boris Johnson. #BadAncient

That sounds like a garbed version of B. Ward-Perkins, The Fall of Rome and the End of Civilization (2005); it follows it (literacy, 'decline' in art, loss of architectural complexity) in order. Personally, I find the last two subjective points the least persuasive of the book.

I wouldn't kick The Fall of Rome out though; the points about measurable declines in nutrition, settlement size and evidence for population are valid and speak to the fact that the collapse of the empire really was traumatic in some places.

One wonders if this strain is particularly strong in the UK because some of the strongest evidence for significant trauma in this period comes from there, to the point where major technologies (ceramics!) were lost (on this see R. Flemming, Britain After Rome (2010)