In the near future the legality of these deferrals will be challenged, but not by suing the President. Instead, the suit will against the employer who illegally failed to comply with ACA.
Eventually some uninsured employee of a firm with 50-99 employees will get in a car crash and run up a $1 million bill in ER. It’s bound to happen. He won’t be able to pay up, and either he, or the hospital, will sue his employer for failing to have mandated insurance.
Whether a Presidential decree has legal authority to override the law will be determined in this manner.
See my post #21 and earlier post #6.