The automated letter part I believe is possible, though I've never heard of it happening to crypto traders (who knows, it may happen).
They seem to have some type of computer system that calculates peoples' taxes and sends out letters if they made any arithmetic mistakes or didn't file a form they were supposed to. And then they'll say if we dont hear back from you then we'll assume you agree with our new assessment. So then you have to call them or write them. Or just pay the bill. I don't know if it happens to crypto traders either.
The audit portion requires actual human intervention. This means resources need to be dedicated and people have to be paid for their work... everyone from the data analyst to the auditor to the collector to the manager. Not to mention travel expenses. Additionally, collectors are often entitled to a cut of whatever they are able to retrieve.
I have to wonder if that's really true that human intervention really plays such a huge role. maybe it used to but it would seem like they are going to start using AI to cut out alot of these data analysis and auditors jobs. they won't be needed anymore. The only thing that will be needed is a police officer to come take you to jail if you ignore their lawsuit.
The letter part is easy. The "going after you" part costs a lot of money.
Imagine one day, AI does everything except rounding people up and throwing them in jail. it makes the decisions about who to audit and how much they owe and even files a lawsuit and presents its own case to a judge. How are you going to win a court case like that?
So no, they won't go after you for a couple hundred bucks.
They might not come knocking at your door but the debt will incur interest and eventually they will either take it out of a tax refund you are due or if it gets big enough, they'll file a lawsuit against you and garnish your wages or sieze your bank account.