The final Republican tax-reform bill unveiled Friday repeals ObamaCare’s individual insurance mandate, leaving the GOP poised to blow a significant hole in the health-care law next week. The change, which takes effect in 2019, removes one of the least popular parts of ObamaCare, but one that many experts warn is necessary to make the law function smoothly. Without a mandate, there is less incentive for healthy people to enroll and balance out the costs of the sick. That is expected to lead to premium increases and could lead insurers to drop out of markets, potentially leaving some areas of the...