I assume you work for the government, and know something about this?
You assume correctly.
The relevant statute is Internal Revenue Code 6502. As it says in the second full paragraph on the linked page,
"...IRC 6502 provides that the length of period for collection after assessment of a tax liability is ten years. The collection statute expiration ends the government's right to pursue collection of a liability."
In practical terms, if your financial situation renders you unable to pay your taxes without undue hardship, an Officer can make that determination and put your account in a "non-collection" status. You still get a letter every year asking if you can pay and reminding you of the debt. Your tax return information gets looked at by a computer each year and if it shows you're making more money, you may get a call. But the IRS has no problem with letting people who truly are unable to pay get away with not paying. It's just a bad debt that eventually goes away.
However, if a person makes enough money to buy a plane and still refuses to keep their tax situation straight, if a person continues to screw up by having "unreported income" in later years, if a person lays the foundation of their tax compliance habits for nearly their entire adult life by getting involved with and buying into the nonsense spouted by what were once referred to as "illegal tax protester" groups, well, then, I feel quite sure that person is going to feel like the IRS has singled them out for punishment and is hounding them for all their life.
That doesn't make it true.
This guy, according to his "manifesto", did all those incredibly stupid things and more. He created his own situation and then he couldn't handle it.
He gets no sympathy from me.
The wife and child he left behind, the workers who lost their sense of physical security at the office, the innocents injured and the one killed - those are the people I feel for.
All that said, I understand that frustration with tax administration is common and perfectly reasonable. I can think of a thousand changes that would make life easier for everyone and make our system of taxation more fair, more simple, and far less onerous.
I assume we are in complete agreement that crashing a plane into a building is certainly not the way to go about making those changes.