Is saying dumb hate speech to people who can't speak

Dude, no.

Hate speech? No. But don't call people dumb. Even if you mean that they can't speak (and for some reason feel the need to discuss that in-game), there are other words that mean the same thing without sharing a word with with a common insult to someone's intelligence.

Typically the term is 'mute' in modern english, and 'dumb' is just a minor insult.

You probably can get banned for it, but it would take a fair amount of effort. Best not to do it at all..

Intent counts for a lot more than what words you use, and the system is pretty good at figuring out when someone is flaming or bothering their team.