I do hear this also, but the only time that nomenclature is used for the '/' symbol, does appear to be when crews report wind velocity (that's both wind speed AND direction).
Personally I REALLY REALLY HATE the sxpression 'slash' as a way of expressing this symbol and I'd like to see a campaign to remove it utterly from our daily lives. The term 'slash' in the UK has unfortunate urinary connotations and otherwise has imagery assocaited with violence and knives. The word 'stroke' whilst not without double meanings of its own is much gentler, invoking images of softness and roundness...
You won't hear UK air traffic controllers saying 'diagonal', though, because it ain't approved phraseology. You'll hear something like 'wind is 230 (degrees) at 15 knots'.
Cheers,
The Odd One