is there a list of opinions that aren't acceptable here?
If I where to take your question literally, and answer it in a pedantic way, I would say "all of them".
This site (and all SE) is not intended for opinions per se. It is intended for actual questions that have actual answers, in the context of each stack (ours is the Professional Life and Environment).
We even have an off-topic reason for questions that are "opinion-based", so you can see that opinions is not what we seek to give to readers and people that come asking for help.
Now, to actually address your concern I will do two things. (1) I suggest you read with more calm the message that the mod team sent you. You will then see that the issue was not about you giving your opinion, but rather the way you worded it (which bottom line, was not nice).
Usually we won't comment on messages that we sent you privately, out of respect for each individual's privacy, but given that you already disclosed publicly that you were contacted I can comment a bit more on that: it was not a threat whatsoever, just a polite request for you to "check yo self before you wreck yo self" ;) and to remind you of being tolerant and respectful when interacting with other users on this site. We expect professional behaviour throughout the site: when you comment, post a question or answer, use chat, etc....
Which leads me to (2), this group of links and references for you to check regarding the expected behavior and conduct of all members of this site (plus a bonus regarding comments):