Yeah it bothers me too. It's not as easy as @[email protected] suggests though, I think. Yes, apostrophe means you're using a contraction, so "it's" should be easy.
But apostrophe also means possession in almost every other case. "It's" and "there's" are literally the only examples I can think of in standard English where an apostrophe can be used, but cannot mean possession. Native speakers still have no excuse, but it is a bit of a weird oddity that has to be learnt.
What really bug's me is when people randomly throw apostrophe's in where they just make no sense, on what should be simple plural word's or even just verb conjugation's. And it happen's all the damn time. (I'm so sorry.)
Agender? I'm looking online now and the closest thing I can find is that some nonbinary people prefer it/its, so that was probably the closest one/best guess from the start.
Edit: Null gender seems like the closest thing I can find, besides the random nonbinaries that prefer it.