The US State Department has listed these countries in part or in whole as Level 3 or Level 4 (there are many others, but these are the ones that pop up on the forum from time to time).
As far as I know there is no law preventing a US citizen from visiting any of them.
I recently visited one of these countries and I did my homework in advance. I discovered that my European Expat Medical Insurance would not cover claims in the country I was visiting. I talked to a friend who checked her US Travel Insurance which also would not cover her in a Level 3 or 4 country. I talked to an insurance agent in the US, and he warned me that I should check my US Medicare Advantage Policy because it probably wouldn’t cover my expenses in the country I was visiting …. But worse …. It might treat anything that happened to me in that country as a pre-existing condition once I did return to the US. That would mean no payment on anything for at least 6 months.
I also checked my life insurance and nope, wasn’t going to pay.
I am certain all of you have much better insurance than I do but never hurts to check before you go. If you do know of a policy that covers Level 3 and maybe Level 4 countries, would be good to know.