Before the ACA insurers could deny you coverage or charge you more for having a preexisting condition like COVID-19. Today you can’t be denied coverage or charged more due to a preexisting condition on any insurance that has to comply with ACA rules. However you can still be charged more or denied for short term coverage AND if the ACA rules are ever loosened, it could leave room for this to happen in major medical plans as well.
Past repeal and replace attempts for the ACA have sought to loosen restrictions on preexisting conditions, although there was always some sort of protection in them (like the ability to keep a plan if you never dropped it), the reality of things is that under looser rules people could find themselves not being able to get covered over something like having COVID-19 in the past and part of keeping the ACA around is ensuring this isn’t the case again.
Learn more about preexisting conditions protections.