If i withdraw my 401k, move the proceeds to an offshore bank, and then leave the country forever, did I just avoid taxes and penalties?

When you "withdraw" that money, unless you are already 59.5 they will automatically withhold a little over half of it to cover penalties and Income taxes....you will get a refund of any overpayment when you file your next Tax return...

If you do not FILE, you will never get that money back.

No.

On a 401k withdrawal, the trustee is required to withhold 20% for estimated federal taxes.

When are taxes due? At the time of withdrawal? And I understand the implications of the scenario would equate to tax evasion... this is just a hypothetical.

No. If the withdrawal is taxable, it's taxable the second you withdraw it, no matter what you do with it. As a US citizen, you owe US income taxes on worldwide income no matter where you live.

Legally you'd still owe the tax. Could you get away with it? Maybe. But remember, they'll have already deducted 20% for tax when you withdrew it.

no you just absconded like everyone else it's called deserting unpatriotic and a dishonour sod it is how you got here