No. It is not illegal, and there has been no federal penalty since 2019. The Tax Cuts and Jobs Act reduced the individual mandate payment to zero for months beginning after December 31, 2018.

Five places still impose a state penalty: Massachusetts, New Jersey, California, Rhode Island, and Washington DC. Vermont requires you to report coverage on your return but charges nothing if you do not have it. Maryland has no penalty either. It just puts a checkbox on your tax return that can start an application.

Being uninsured is legal. It is also the reason medical debt is the leading cause of personal bankruptcy in this country. And if you skip coverage now, you generally cannot buy it mid-year when something happens. You would be waiting for the next open enrollment.