I thought some Scottish matches were covered possibly in the very early season or last season not sure what competion though.
If you are refering to the English Football Conference National where many but not all matches are on the 4K channel that league tends to be very flexible as to when they reschedule those games so they enable BT to broadcast them at times that suit them.
From what i notice Scottish Football often tends to be scheduled (eg on Friday nights) when BT are broadcasting the rugby in 4K and there is only one 4K channel.
Another consideration is whether the Scottish Fopotball is filmed by a unit that has 4K cameras or not.
I notice that at least some of the BETFRED Cup Scottish football is listed on the BT Sport 4k UHD channel.
For example the Hearts v Dunfermline is currently scheduled for a delayed broadcast at 8:15 pm Saturday ... the live match is on the BT Sport 1 channels during the afternoon ( the 4K channel has the rugby 7' on at that time., pity there isn't a BT Sport Extra 4k channel).
The Dundee v Dundee United game is scheduled live on the Bt Sport 1 and 4K UHD channel on Sunday afternoon.