Compared with other provisions, the young-adults requirement proved fairly uncontroversial among insurers. Many even volunteered to comply well before the deadline set by the law, noted Robert Zirkelbach, spokesman for America’s Health Insurance Plans, an industry group.

In addition, nearly all states, which regulate many forms of private insurance, have already codified the young-adult rule at the state level. In some cases this was done through actions that could be easy to undo. For instance, South Dakota’s law adopting the young-adults requirement included the proviso that if the health-care law is found unconstitutional, the state statute would automatically be repealed as well.

But in plenty of other states, insurers would not be free of the rule unless state leaders rolled back the statutes or regulations they adopted to implement the health-care law.

The same is true of the host of other mandates the federal law currently imposes on insurers. These include prohibitions against imposing lifetime limits on insurance payouts or dropping someone’s coverage after they get sick on the grounds that their insurance application contained inaccuracies.