"see it in print" can be "see it".
"Or more realistically in online posts" is also covered by "see it".
Or "drives me nuts" alone, because seeing it and where you see it are implied because "drives me nuts" requires you to have interacted with it somehow.
"the only legitimate usage" can be "the only use", since you wouldn't be writing about illegitimate uses.
"to put it into the mouth of a character who you want to sound pompous" can be "it sounds pompous".
"whatever usage we’re talking about" can be "the usage".
"there's no ambiguity in the shorter form" can be "there's no ambiguity".
"Just cut it out" can be "cut it out".
"don’t wedge it into your prose" can be "don't write it".
"you should probably be nicer to your friends" can be "don't".
"In normal conversation [...] in normal conversation" is redundant.
Let's see how that comes out:
> "'Aforementioned' drives me nuts. It sounds pompous; it is redundant. For example “The aforementioned usage is …” becomes “The usage is …” - there’s no ambiguity. Cut it out. If you wouldn’t say “aforementioned” in normal conversation then don’t write it. If you would, don't."