Pre-ACA, guaranteed issue existed for employer group plans but not individuals. That means someone with a pre-existing condition could not be excluded from joining a group plan, but could be denied individual insurance. Insurers were required to offer plans to businesses. They did have the flexibility to limit offerings to “very small” businesses, so self employed people usually found themselves excluded from business insurance options.
When someone covered under a group plan left it, insurers were required to offer a transitional plan that was guaranteed renewable. So, leaving an employer plan, one still had one option to obtail individual health insurance. There were few regulations regarding coverage or cost, so there was no guarantee it would be affordable or comprehensive, and often had limited network options.