Prescriptions and descriptions are not opposites. They're orthogonal to each other:
when you tell people how things are, you're being descriptive;
when you tell people how things should be, or what they should do, you're being prescriptive.
And prescribing is not automatically wrong. For example if I were to tell someone "don't call us Latin Americans «spic removeds», it's offensive", I am prescribing against the usage of the expression "spic removed"; it is prescriptivism. Just like when someone proposes inclusive language.
What is wrong is that sort of poorly grounded prescription that usually boils down to "don't you dare to use language in a different way than I do, or that people did in the past". It's as much of a prescription as the above, but instead of including people it's excluding them.
Tagging @[email protected], as this addresses some things that they said.
This is fair. Usually when I hear "prescriptive" I have a knee-jerk reaction to it as something bad because it's usually used to refer to people using made-up rules to enforce systems of oppression rather than fight against them like inclusive language does, but I hadn't thought about it as "prescriptivism for good."
The knee-jerk reaction is understandable, since most prescriptions are of the exclusionary type. And at the same time, since linguists say "we're describing, not prescribing", people create a false opposition between both things. And, well, if description is scientific and good the prescription ends as "unscientific and bad", through that opposition.
Using inclusive language isn't linguistic prescriptivism. Prescriptivism is saying "this word is incorrect English/doesn't mean what you are using it for." Inclusive language is saying "if you use this word, you're being a jerk."