Mallaig/Kyle of Lochalsh is an unlikely point-to-point rail journey, requiring travel via Glasgow.
The OP's question is slightly ambiguous. The thread title says "stations nobody has ever made a journey between", whilst in Post #1 OP asks "any stations that have received zero ticket sales to certain other stations". These aren't quite the same thing.
I'd bet somebody, sometime
has travelled by train between Mallaig and Kyle of Lochalsh via Glasgow. If I were wandering around Scotland on a Rover ticket, it's the sort of trip I might make myself.
Agreed it's unlikely a "normal" A to B traveller would ever do it, although there is a published fare: £132.40 Anytime Day Single.
With some of the better known pointless journeys (e.g. between the two St. Budeaux), at some time, some enthusiast with a bit of spare time on their hands has very likely done it.
Plus there will be many more random & unpredictable unlikely pairs which have seen a traveller or two due to unique circumstances with family, friends, work etc. Some examples are posted above - my own is Hindley to Datchet.
Of course there are still very many combinations of small, run-of-the mill stations separated by medium or long distances which genuinely have never had anyone, rail nutter or otherwise, travel between them. And a good few combinations of origin & destination never travelled due to railway geography - like Fenchurch St to Penzance, or London Marylebone to Margate, or London Waterloo to Manchester Victoria (although Vauxhall to Salford Central would be quite plausible).
Someone with a better grasp of combination & permutation calculations might correct me, but if there are 2,566 National Rail stations, this means there are 6,581,790 possible journeys (counting A to B and B to A as two separate journeys, half that number if you consider AB = BA)
If 99.5% of these journeys have actually been done at least once at some time (this is probably too high - dream up your own estimate if you like), that means around 33,000 pairs of stations which nobody has ever travelled between.