many of the most core US IT infrastructure projects- government and commercial- depend on IBM 360 assembly code or another IBM system, written decades ago that has been repeatedly scaled to handle the increase in load.

Two I can think of: SABRE and the IRS. See more here: https://www.nextgov.com/cxo-briefing/2016/05/10-oldest-it-sy...

Personally I have a ton of respect for production systems that can be scaled in place for 50 years. But I don't have any interest in learning to run IBM mainframes or code for them- I'm preparing for lots of contract work in 2037.