I used to behave like this, and it's not very helpful, and usually turns it into an argument. Just silently ignoring it works much better.
I put my foot down on that one - called the initiative ableist right in their “party” channel. And stated that if my participation was an issue, then I’d like to request non-participation as a reasonable accomodation for my autism.
In your example - marketing sends a message on slack to post something on LinkedIn. You can:
- Just do it
- Just not do it
- Not do it and be (very) vocal about not be willing to do it
Picking the last option and complain is probably the worst thing you can do. You just open a can of worms, and - especially if you do it in a public channel - you put them in a position where have to be defensive or explain themselves.
Basically instead of taking what they say at face value "this is what you must do" - as a real concept - so "therefor you must defend yourself and try to get out of "doing what you must do"" by complaining against it - take a step back before even considering that is really something you must really do. It's not. So just don't do it... but don't throw it in their faces that you're not gonna do it. Haha
Like 99% of these things - if you just silently ignore them, they'll just go away without a fuss.
It's not your problem if you don't do it, so not even worth trying to argue over. It's their problem. And if they think it's a big problem enough they'll probably send some more reminders in public first - like "We see not a lot of people have posted on LinkedIn! Please do, it's very important." - still just ignore it. If at some point they start DMing you about it, that's about the right time to put your foot down and directly tell them you're not going to do it