For example if using "slave/master" terminology becomes widely accepted as insensitive, in the future someone may look through my twitter history or something and find that I had used that term, even though at the time of writing no one had considered that term insensitive.
This might happen occasionally with words that become offensive, but I'd argue a lot of such words that "were not offensive" actually were, it's just the author was oblivious. In such cases, you'd hopefully be able to argue that the author was unaware.
Although some people might feign ignorance as an excuse when they very definitely knew what they were saying.
> For example if using "slave/master" terminology becomes widely accepted as insensitive [...]
> [...] even though at the time of writing no one had considered that term insensitive.
Well, it always has been insensitive, but the vast bulk of the tech industry has been people who've not experienced slavery (or didn't consider the implications for those who have), so they were oblivious.
> in the future someone may look through my twitter history or something and find that I had used that term
Well, you might not have realised. Someone might call you a monster, but I think people would be somewhat forgiving if you hadn't yet been warned.
(This does depend on severity, though. With some words, obviously it's impossible to argue you couldn't have known.)
First off, if you think that the sort of folks who make up online mobs are inclined to be forgiving under any circumstance, please let me be the first person to welcome you to the Internet. You're going to be surprised by a lot of things during your stay here.
But more importantly: "warned"? Who gave these people the authority to "warn" people about what vocabulary they use? And do other small and frequently mocked subgroups of society, like, oh, say, Mormons, or Southern Baptists, also get to "warn" people about insensitive remarks, and have their warnings automatically obeyed? If not, why not?