It’s not entirely clear that ‘overmorrow’, as such, was ever really a widely used word as opposed to an at best half-resurrected invention, and evidence to the contrary is extremely hard to find.

I see an unsourced reference to Middle English ‘overmorwe’, but I can’t find anything specific until the Coverdale Bible of 1535, written by one man, who had been exiled in Antwerp, at a time when Continental influence was strong. I can’t find any evidence of the use of its Germanic cognates prior to the proliferation of Low German vocabulary via the Hanseatic League in the High Middle Ages, so it may have been a Low German innovation that spread to High German (or vice versa), then Scandinavia, the greater Netherlands and - possibly via Coverdale - English, as a ‘calque’, or direct translation of parts of a compound. This was also an era when Dutch printers were very influential in early Protestant England and had the affect of adding some words and novel spellings (starting with the reign of Henry VIII).

Evan Clifford Llewelyn’s 1936 “The Influence of Low Dutch on the English Vocabulary” also cites Coverdale and suggests a probably Dutch or German origin as opposed to an Old English one.

It appears in dictionaries only in the later 19th-early 20th century, at least for the OED and Webster, and Google Ngrams (which measures frequency of words in the major repository of data that is the enormous English corpus available to Google Books over the centuries) only shows a few brief spurts of usage in the 1800s, and the examples I have randomly looked at mostly seem to be discussing the word rather than just using it, and another one I see deliberately trying to imitate an affected old-fashioned style (even then).

So based on this, I would say we can’t be sure that this word was ever truly on common use (I’d be inclined to say it wasn’t), rather than a particular calque from Dutch or Low German used by a few writers and then occasionally brought up as a ‘cool’ curiosity. This raises the question of to what extent it can be considered a ‘true’ English word. It certainly has more traction in other Germanic languages, but may not have been an ancestral Proto-Germanic word: many people often don’t realise that many of the more complex cognates between German, Dutch and Scandinavian languages are not simply due to being Germanic, but late medieval/modern semi-literary borrowings.

Very similar seems to apply to ‘ereyesterday’.

English linguists may be able to answer this better, so I’d also try r/linguistics and r/etymology.