I don't think there's anything that should be considered "off limits" in terms of how you want to portray your character(s).  

But in terms of "how you manage them in roleplaying or collaborative writing?"

My only true advice, is that giving your characters disabilities is a writing challenge that [b]should[/b] only make it harder for [b]you[/b]. (A personal flex to show how well you can portray said character.)

So, if your character doesn't speak. (Or speaks rarely in stressful situations - that let's be honest - good roleplays will likely put them in.) Then it's up to you (your character) to effectively communicate without using words. Whether its more focus given to facial expressions or body language. Or maybe using a gimmick that shows that the character is clever and pro-active in mitigating said disability. (Knowing how to sign, carrying a journal to write in, or maybe wearing something differently to show that they're too stressed to verbally communicate?) 

Anything like that, will surely make your character far more likable and sympathetic.

Because the alternative answer is to make a character that's effectively an anchor to progress. A typical one I see all the time is the "shy" character archetype, which effectively means having literally no agency for themselves. So they almost never contribute or act first in the narrative. And that's not fun for anyone else.

So, try your best to not make a disabled character 'a Mary Sue' or 'a Damsel in Distress'. You should [b]show[/b] the struggle they're going through. (Maybe their usual workaround doesn't always work.) But your character shouldn't act like a puzzle or trap that forces the other players to adapt to them.

An example of "mutism" you'd want to avoid that happened to me; is my character was trapped and had to somehow communicate with someone else who was mute. But when my character found parchment and ink to write questions down for them "to shake their head yes or no" to. My writing partner didn't seem to realize that was an option, and never really had effective way to handle it. Because "not speaking to them" was the entire point of their character. And I found an impossibly simple and obvious workaround...

[i]Never thought to just make the character illiterate either.[/i] But yeah. If every single interaction with the character goes down like "Bob listens to the frantic questions of his peers. All sinking into a strange quicksand that only Bob knows how to escape. So Bob looks away and says nothing. His disability is very traumatic. Woe is he." People will be more annoyed at *you the writer* over Bob's accurate portrayal of a guy you might know.