What's legal and what's enforceable are two different things. Also the US is extremely muddy in these legal waters because, sure, Federal laws and the Department of Labor are supposed to enforce such actions. But most US states are "At-Will Employment" states which means that companies do not even have to provide a reason for a termination, which means there's no paperwork that really gets sent to the Department of Labor to enforce anything at all.

(Not to mention that companies have spent decades perfecting the "layoffs are coming in a quarter or two, send out performance reviews now and encourage managers to be extra 'honest' and nitpick everything so that we have a written record just in case we need to fire people for performance to make the layoff seem smaller" dance, including Stack Ranking being built to be that smoke 100% of the year, every year. Still technically illegal, but if the quiet parts aren't said out loud who are you going to believe, the months ahead of time paperwork in performance reviews or that executives decided they needed harsh performance reviews months in advance on a golf course somewhere? It's not a conspiracy when it's so easy to pull off.)