Unless they've already found something suspicious, in which case for certain things they can go back forever to rack up the criminal charges.
That's part of why it's important to destroy documents when you are legally able to. If you keep those documents longer, they are a risk to you in case of a "every tax return ever" investigation. If all evidence has gone, as you were legally allowed to do, there is very little chance they'll convict you for that stuff.
Even if you aren't a criminal, some innocuous things you do today might become illegal in the future as tax law is refined through case law.
Are you suggesting that things that are legal now, subsequently deemed illegal in the future, could then be retroactively brought against you? That sounds... incorrect.
He's not saying the law will change retroactively. Just that future interpretation of tax law via case law could change what was previously thought to be legal.
That's part of why it's important to destroy documents when you are legally able to. If you keep those documents longer, they are a risk to you in case of a "every tax return ever" investigation. If all evidence has gone, as you were legally allowed to do, there is very little chance they'll convict you for that stuff.
Even if you aren't a criminal, some innocuous things you do today might become illegal in the future as tax law is refined through case law.