I don’t disagree with a thing you said, but it’s a complex environment. There are words no one can use, words only certain people can use without offense, words that offend only in the wrong (potentially misunderstood) context, some people care about pronouns, some don’t. My neighbor will be very brusque even with total new acquaintances if you slip up and call him Dave instead of David. Etc.
I try my best to avoid words or symbols or actions that might upset someone because as you say, zero cost (not really, because you have to pay attention and track all the evolving public zeitgeist about a bunch of stuff, but, still, a fair enough point). But I also think there’s some responsibility on others to have grace as much as they want people to be tolerant toward them. It costs them “zero”, also, to assume any mistake is an honest one, absent real evidence to the contrary.
Fortunately that usually isn’t a problem in real life. It’s only on social media where I’ve ever seen it be an actual significant problem, due to what one wag some years back termed TIFS – Total Internet Fuckwad Syndrome. Take an ordinary person, give them a bullhorn and anonymity, and they get to take up offense at the slightest thing AND be a total asshole about it.
Sometimes people forget that they don’t have to have 110% of their social capital online.