They probably did at an extremely high level, I already paid and threw it away (which was silly... don't do tax stuff when mentally exhausted) but it certainly had no specific details or context on how they arrived at that conclusion, what exactly was wrong, etc. It did have some very serious statements about failure to pay, increased penalties for late fees, etc. and having tried to call the IRS in the past, I know that it's worse than pulling teeth to get real answers so I decided this fight wasn't worth it. That said, I have also had them come back before with a mistake on their end and a check I wasn't expecting so my current mental model for the IRS is that they are honest brokers in general, just really really slow.
They will say what for and give you the opportunity to dispute it. You can even call and talk with someone. They aren't faceless, heartless, or infallible. If it's their mistake, or an honest mistake on your part, they're pretty easy to work with.
