'Diagonal' has been common, indeed standard, usage in all my 34 years professional aviation.
...and in my 30 years I've never heard it. Of course, I'm happy to admit that that may be because I only spent the first six or so flying around the planet, and the remainder atc-ing, so I've lived a sheltered life.
But to the actual reason for this missive... we here on this side of the pond (well, in middle part of the upper part of this side of the pond anyway) frown upon "slash" as well, using the word "slant" instead. However, usage is pretty much limited to aircraft type and equipment (I have a theory that new pilots are obliged to use the frowned-upon term "slash" as in "we're a Piper Cherokee PA twenty eight one-eighty-one Archer two slash alpha..." until they've accumulated 75 hours, and then must never speak it again). I've never heard "slash" or "diagonal" with regard to wind. There, it's always been "at."