An interesting take I hadn't considered.

From my perspective, "please calm yourself down" isn't designed to tell the other person how to feel but how to present themselves when they interact with me. But yeah, I see how it could be viewed from that perspective.

Part of my reasoning is be who I've had yell at me in the past when I haven't done anything to deserve it (family, colleagues). In those cases, I expect them to maintain a civil tone, which isn't yelling imo. Honestly I don't really care if they're boiling with rage underneath but I expect them not to abuse me if they want to continue interacting with me.

I have the same issue with freezing (or apologising on repeat even when it's not my fault), especially in situations where I'm tired, low on spoons or (annoyingly!) in the workplace. However as far as I'm concerned, once someone is communicating with you by yelling, that's their problem, not yours. They're the ones who have caused the communications channels to break down & they're being abusive.

My advice is to explain that you're walking away from the situation & do exactly that (similar to a customer service rep hanging up on an abusive caller). You may need to develop & practice your explainer until you've got it flowing smoothly so you can just deliver it even with your mind frozen. If you can't deliver it in that state, don't worry, just walk away.
Keep your explainer short, a sentence or two at most (e.g. I'm walking away now as you're yelling at me. Please calm yourself down if you want to talk to me again.). Do not attempt to engage them further & keep away from them until they've got control over themselves.
Then make sure you get somewhere safe with other people around as social protection if the person yelling tries to follow you or escalate.

I'm new to Kbin (<24 hours) although I've been on Mastodon for a while. Coming from Reddit, I found this to be a useful introduction.