Some general principles about saying slur words:
If something is a slur for a group you are not part of, it is not ok for you to use that word in conversation (and proceed with extreme caution about writing fictional characters who use that word). You should not call anyone else that word (even if it’s how they refer to themselves), and you should not call yourself that word. And you should not use that word to make a point of how marginalized your group is.
Some words are slurs some of the time, but not all of the time. For instance, someone who is Jewish is a Jew, and calling them a Jew is not a slur. But if you say, “Oh, you’re such a *Jew*” to someone who you think is being unduly stingy, that *is* a slur, and it’s not ok.
Some words used to not be slurs, but are now. So you shouldn’t say them, but you also shouldn’t be horrified if you find them in old books or articles, or if elderly people still use those words without intending a slur. Other words have *always* been slurs, and you *should* be offended if old books or elderly people say them. But I’m not sure how to say which are which without hurting people, and some of the lines are blurry.
Some words are considered impolite by well-meaning people outside a group, but are actually the preferred terms people use to refer to themselves, and you should use those words. For instance, some non-disabled people think that “disabled” is offensive and that people should say “differently-abled”, but most people who actually *have* disabilities find that annoying or offensive. (Likewise “autistic” vs “person with autism”).
This is different from reclaiming slurs – some people use slur words to refer to themselves as a way of dealing with the pain those words cause, but don’t want other people to use those words. It can be hard to tell the difference, sometimes. If you make a mistake, apologize (and don’t make a big deal out of explaining what you Really Meant, it will probably make things worse).
Some slur words are widely used by people who don’t realize they are slurs, and who have no intention of insulting or expressing bias toward anyone. It’s still not ok to use those words, because the people they apply to *do* know they’re slurs, and they’re still hurt by them. But there is still a difference between people intentionally using slurs to insult people, and trying to use the right words but failing.
Also, some people with communication disabilities can’t change the words they use, or can’t do so and still retain an ability to communicate. This doesn’t make actual hateful speech from them ok, and it doesn’t render things harmless, but it’s important to be aware that this is a thing and not treat using correct language as an absolute precondition to being included in a space.