No restrictions, really. I visited Cozumel (again) at the end of August and the only annoying thing was that I needed to send my phone to a link and answer covid-related questions, and then show the result for an inspection. The guys at the Cancun Airport didn't even pick up my form. They did ask me some covid-related questions, and I just smiled and said no to all of them. Then they smiled and didn't bother me further.
Mexico is happy to have American tourists. Just play along with the mask Kabuki. Your temperature will be checked, and you'll need (idiotically) to step on a alcohol-infused rubber mat when you enter most places. (Suddenly the WuFlu is foot & mouth disease!) But you won't be pestered too much beyond that. Restaurants just want you to wear a mask to you table.
Speaking of restaurants, I'd like to plug La Cocay. Chef is excellent, especially with duck. Last time I asked for rare duck because the time before that it was medium. Maybe medium-rare sholud be specified, but I erred on the side of "rare." Sure, some of it was closer to raw, but I don't care. It was glorious!!