Remember those really annoying Royal Bank of Scotland £1 notes that they kept producing more than a decade after coins were introduced?
So you'd have maybe forty pounds worth of £1 coins, and maybe three or four of the RBS £1 notes
Then when they gradually disappeared, you'd go days without seeing one, then suddenly someone would pay you with a bundle, and that's all they had
According to Wikipedia, they're still around (RBS just stopped producing them) but it's years since I've seen one
Of course, even though the Scottish banks change their notes (for the plastic ones, most recently), I don't think they ever actually stop being legal tender like the Bank of England ones. They just gradually withdraw them from circulation as they're returned to the banks.
But that's maybe something to do with the fact that the Scottish notes aren't ever legal tender as such, in fact *no* banknotes are legal tender in Scotland
So in a way Scottish notes are no more legal or illegal than the $, € or chickens or goats as a means of payment - it's up to the driver what to accept.