Penalties include garnishment of wages, bank accounts, and future Federal tax returns until the debts are paid off. There’s ways around it - self-employment can prevent garnishment of wages, bug not of bank accounts, and making sure you don’t overpay on taxes so you aren’t owed refunds will prevent garnishment of those.
Your credit will take a hard hit. Forget getting credit cards, mortgages, etc. for years. If you’ve already got a mortgage, you won’t be able to refinance.
Other than that, it’s going to depend on your locality. Some states, you can lose your driver’s license. Debtors prison is technically illegal - debt is a civil matter not a criminal one - but don’t be surprised if that changes.
Debt collectors will hound you relentlessly, and will hound your family members as well, looking for you, which is barely legal, but they get around it.