It always has been (I say ‘always’… since 2002 at least). There are four restricted areas, R610A-D, covering a lot of the Highlands for military training flights.
It used to be active Mon-Thu 2pm-10pm/3pm-11pm (summer/winter), but looks to have changed to be activated by NOTAM.
Search for R610 in ENR5.1 for the details (and the contact number if you’re looking to fly there when it’s active).
I’d say if there’s no NOTAM then you’re fine, but give the number a ring if unsure.
As a semi-interesting aside… I grew up in the Highlands and the Harrier pilots used to use my primary school as the target for their practice bombing runs two or three times a month. Not sure they’d get away with it now, but it was pretty awesome.