My mom, who is currently 45, says it a lot. But maybe it has to do with it being more common with non-north american english. Yahtzee is a Brit and my mom learned english from a south african.
In fact, I'll bet that that's it considering that, as you said, apropos comes from French and British English has always pulled from French starting from the Norman invasion to the majesty of Louis and the success of Napoleon. Its why, if I remember this correctly, the "posh" accent was created. It sound more French, particularly the "r"s.
There's nothing I hate more than people freaking out at words over 2 syllables long. I take philosophy in university and the crap I have to read has maybe desensitized me to complex phrasing. Its very different from regular writing because you want to say things in the least amount of words, not with the most coherent words.
And in terms of "slang", then we get into a discussion of prescriptive and descriptive definitions for words. I don't think you can win arguing for prescriptive because then it fails to explain how words are used, implying that all slang is meaningless or only understood within a unique community. But Britain is not a community unique enough for that, and it could be understood by those outside of the community. The only reason to dislike slang in writing is twofold: ambiguity. Sometimes the meaning of the slang is too specific but would appear to have a more general application or just a different way of using it. This would make it difficult for people who aren't used to the slang. Two, you are an extremely prescriptive grammarian and so certain things should be avoided. However, consider that if you go to Webster dictionary, even they would support slang usage of terms both in the dictionary and in their special video series (where they taught me that ending a sentence with a preposition is not as wrong as people think it is because prescriptive grammar was mostly codified in the 18th and 19th centuries and was even then not near being descriptive of actual language use).
If I wrote an article about hipsters, would that be slang or a word? How does something go from slang to word? How do you even measure such a thing? I don't think you can but I'll be interested to see what you have to say on the topic since all I know is from Philosophy of Language and you seem to have more knowledge about general writing.