However, I believe Ms Morrison is mistaken when she says that “the shutters came down” on Scots after the 1707 Union. This happened earlier.
R W Cochran-Patrick’s volume Early Records Relating To Mining In Scotland gives reproductions of documents in Scots from the 16th and 17th centuries. After 1609, that is six years after King James moved to London, Scots words containing quh (quhilk, quhen) are gradually replaced by English forms using wh (which, when). Similarly the Scots past tense ending in –it is replaced by the English –ed.
Thus by the early 17th century, Scots as a written language had not so much died out as merged with standard English. Sir James Hope in his 1646 diary of his visit to Europe uses very few Scots words. I would suggest that the Cromwellian occupation probably brought to an end the use of Scots for serious written communication.
Of course, Scots as a spoken language continued until after the Parliamentary Union, as it still does to some extent today.
Richard A A Devéria, Aberfeldy