One of the corrections in the second edition of Jane Austen’s Mansfield Park, originally published in 1814 but reissued in 1816, includes you was. This change was recorded by Kathryn Sutherland in her comparison of the two editions of the novel (Penguin edition 1996:415-424). The speaker is Fanny Price, who said “I thought you was against me”.

Henry Churchyard, on his Linguistics Page already drawn upon elsewhere in this blog, notes that he only found four instances of you was in the novels:

the phrase “you was”, for which the statistics are Narration: 0, Lucy Steele: 3, Nancy Steele: 1.

For his analysis he notes that he drew upon freely available online editions of Jane Austen’s novels, and the version found in Project Gutenberg indeed shows the correction of you was into you were. There should therefore have been at least one more instance. (Jane Austen herself doesn’t use you was in her letters.)

You was was first condemned by Robert Lowth in his grammar first published in 1762 (Tieken-Boon van Ostade 2002), and as I show in my article, usage dropped significantly after that date. The reason why Lowth included a stricture on you was in his grammar may well have been that usage at the time was so frequent that his attention as a normative grammarian was drawn to it. This is how usage items frequently attract criticism. The split infinitive, first commented on in the early 1830s (Bailey 1996: 248), is a similar case, and so is the modern use of like, though I haven’t come across any comments against it in modern usage guides.

We are looking for publications on you was in the history of English. So far, I know of one article, Laitinen (2009),  in addition to my own. Is anyone familiar with any more publications on the subject?

References:

Bailey, Richard W. (1996). Nineteenth-Century English. Ann Arbor: University
of Michigan Press.

Laitinen, Mikko (2009). ‘Singular YOU WAS/WERE variation and English normative grammars in the eighteenth century’, in Arja Nurmi, Minna Nevala and Minna Palander-Collin (eds.), The Language of Daily Life in England (1400-1800). Amsterdam/Philadelphia: John Benjamins, 199-217.

Tieken-Boon van Ostade, Ingrid (2002). ‘You Was and Eighteenth-Century Normative Grammar’, in Katja Lenz and Ruth Moehlig (eds.), Of Dyuersite & Chaunge of Langage: Essays Presented to Manfred Goerlach on the Occasion of his 65th Birthday. Heidelberg:
C. Winter Universitaetsverlag, 88–102.