I've run across this in the past, and found 2 examples again today.
First example: How do I help my itchy Bichon Frise? Comment: "OP, please calm down, take a breath and read what you wrote."
Second example: Cat's behaviour when dying In answer: "Relax" (I edited it out, see revision history).
In online communication, telling people to relax is not a good thing, and it's something we should discourage our frequent answerers from doing.
First, the questioner may not be excited, just bad at writing. I think we see a lot of this, people write a single paragraph with disjointed thoughts, no capitalization or punctuation. I've spoken with people who write like this, and it generally doesn't indicate excitement but generally either an unfamiliarity with complex writing tasks, or a lack of concern with online standards of discourse.
If it's a situation where you've talked with a person through written media often and the format/disjointedness is unusual, then you may have cause to believe that the person is excited/upset. We just don't have that context in many of the questions we get here.
Secondly, even if the person is excited/upset, an imperative command is not going to reduce their anxiety. I found a good explanation of this problem:
That’s because you are dismissing the gravity of what they are experiencing. You are condescending them by telling them how they should feel. Attempting to alter their experience in a moment is suggesting that they are not to be trusted with their emotions. Ugh. Unhelpful and frankly, annoying.
So stop doing it.
Finally, this is often a gendered problem. I'm not going to get more into that here, but it's another reason to knock it off.
tldr:
I believe we should stop telling folks how to manage their feelings and just answer their questions about their pets.