Not a mod, obviously, but I think a good rule of thumb is to simply think about whether your prompt/fill has language and/or themes that could be found offensive and think about what warnings you want to apply to it. Granted, yes, it's hard to warn for every possibly offensive thing (because, you're right, it's a kinkmeme, and there's always going to be something that someone could find upsetting or offensive or objectionable), and you can't always predict what will or won't be objectionable to other people. But it doesn't seem to me like the mods are going to indiscriminately delete every prompt or fic that has the word "cripple" in it or depicts Charles as uninjured post-movie - rather, it sounds like they're asking simply that we take some care with our warnings and accept that they may ask us to modify our warnings if they come across something they deem unacceptable.
I understand your concern about policing and censorship, but the thing is that, for some people, reading something offensive is not as simple as "exiting out" and leaving it at that (oh lord, would the internet be a simpler place if it were!). For some, it's not the same as "I don't like slash" or "het really isn't my thing" or even "I'm squicked by noncon," and scrolling past, but that reading a prompt calling Charles a "cripple" or the like can significantly affect the fandom experience. Of course, that's not the case for everyone, and not even the case for all who have a personal connection to Charles's disability. But that's why the mods are not telling anyone what to write or not to write in their prompts/fics, but instead asking that we give a warning for language and themes that might be hurtful to others, just as we would for themes of rape and violence and incest and underage and the like. You're right that there is really no clear cut sense of what is and isn't offensive, and no one should be forced to take a stand on something that they don't agree with, but using a warning along the lines of "may contain objectionable/offensive language" serves well, I think, to cover your bases if you think someone could take offense.
I think the "choose not to warn" idea is a very good compromise, and one that seems to work well in fandom in general - this way, no one is forced to use warnings that they don't agree with or feel could spoil the plot of a story, and those who know themselves to be sensitive to specific themes and language can consider whether to read or not.
I agree that this post is a little clunkily written, but I do appreciate that the mods are open to discussion on this and amending rules as they go. Nothing in fandom is ever perfect, things are always changing and evolving as time goes on, and I think the most important point here is to try and reach a compromise that benefits everyone, that allows everyone to have the best possible experience here. So really, the most important thing here is to just think about how your words might affect someone else and think about the most appropriate warning you can give to others, even if it's "I'm choosing not to warn."
Re: Mod Post
I understand your concern about policing and censorship, but the thing is that, for some people, reading something offensive is not as simple as "exiting out" and leaving it at that (oh lord, would the internet be a simpler place if it were!). For some, it's not the same as "I don't like slash" or "het really isn't my thing" or even "I'm squicked by noncon," and scrolling past, but that reading a prompt calling Charles a "cripple" or the like can significantly affect the fandom experience. Of course, that's not the case for everyone, and not even the case for all who have a personal connection to Charles's disability. But that's why the mods are not telling anyone what to write or not to write in their prompts/fics, but instead asking that we give a warning for language and themes that might be hurtful to others, just as we would for themes of rape and violence and incest and underage and the like. You're right that there is really no clear cut sense of what is and isn't offensive, and no one should be forced to take a stand on something that they don't agree with, but using a warning along the lines of "may contain objectionable/offensive language" serves well, I think, to cover your bases if you think someone could take offense.
I think the "choose not to warn" idea is a very good compromise, and one that seems to work well in fandom in general - this way, no one is forced to use warnings that they don't agree with or feel could spoil the plot of a story, and those who know themselves to be sensitive to specific themes and language can consider whether to read or not.
I agree that this post is a little clunkily written, but I do appreciate that the mods are open to discussion on this and amending rules as they go. Nothing in fandom is ever perfect, things are always changing and evolving as time goes on, and I think the most important point here is to try and reach a compromise that benefits everyone, that allows everyone to have the best possible experience here. So really, the most important thing here is to just think about how your words might affect someone else and think about the most appropriate warning you can give to others, even if it's "I'm choosing not to warn."