Descriptivist language can absolutely describe things like "ain't", by saying "here's what it means, and also here's how you'll be perceived if you say it". Similarly, descriptivist language can tell you what people mean when they say "irregardless", and also tell you that it's commonly perceived as incorrect usage. (That's more useful than just saying "that's not a word".) You can look up a slur in the dictionary, and you'll find it there, along with some history and context and how the word comes across to people. Descriptivist language includes "how formal is this", "how offensive is this", "how correct is this", and so on.

It's useful, especially for people who aren't already steeped in cultural norms, to have a reference for "how might I come across if I say this". Words simultaneously communicate meaning and connotation, and it's helpful to understand both the meaning and the connotation. People aren't going to misunderstand the author of a text that says "blacklist"; they're going to understand just fine, perhaps including ways that the author would prefer not to have been perceived, or perhaps in ways the author intends to be perceived.

Tooling like this won't change the minds of people who are determined to be offensive (with or without the added assertions that they don't think it should be considered offensive). The point is to inform people how they may come across. It would be incorrect to say "this word is universally considered offensive", just as it would be incorrect to not label the word at all; it'd be more correct to say "this word's status is [disputed/transitional], with [an increased trend towards being considered non-inclusive], and consequent [doubling down by conservative language users]; consider avoiding due to any or all of offense, controversy, or politics".