I had understood that there was some kind of Brythonic language attested in the Scottish Borders.
Cumbric? At least until the victory of Anglo-Saxon languages there three or four centuries after the Roman withdrawal.
So yeah, not quite (modern) Scots Gaelic but part of the same family of languages. And not at all recently, but potentially hundreds and hundreds of years of language contact in domestic and community use in some parts, as early Scots was first forming out of a continuum of Northumbrian English dialects.
The problem is the extremely sparse written record.
It's basically "pre-history" in terms of academic discipline, until the high medieval. There just aren't historical/written sources.
What is known comes from third party accounts and relates more to current political happenings than language, history and whatnot.
A Saxon scribe describing the politics of Scotland with 3 sentences of Latin doesn't tell you much about Scottish peasantry.
Even into the early modern, it's pretty hard to know what's happening with spoken dialects.
We know Romans used "Scot" referring to Gaelic speakers, the language spoken throughout Ireland.
We know that 1000 years later Constantine, the product of Pict-Scot marriage declares a united kingdom of Scotland. We know that proto-english, danish, Gaelic and (presumably) Pictish were spoken. What writing exists is usually in Latin.
That said... who knows. The academic "best guess" is not necessarily a good guess.
So yes... Pictish might be closely related to Welsh. It might have been a totally unrelated language. It might not be a language at all.
Also there's a lot of baggage. Ideas from shaky 18th century sociology might be crucial to modern identity.
The idea of Scotland as a uniform "Gaelic kingdom" is a highly political one today.
Cumbric? At least until the victory of Anglo-Saxon languages there three or four centuries after the Roman withdrawal.
So yeah, not quite (modern) Scots Gaelic but part of the same family of languages. And not at all recently, but potentially hundreds and hundreds of years of language contact in domestic and community use in some parts, as early Scots was first forming out of a continuum of Northumbrian English dialects.