This mostly affected the individual and small-group markets, in which a pre-existing condition either disqualified you for coverage, made it prohibitively expensive, or explicitly excluded coverage for that condition (sometimes, the exclusion was only for a set period of time).
Typical failure modes were: individuals unable to get coverage on the open market; small businesses and non-profits that offered employees health insurance being forced to fire people because they got expensive illnesses and their insurance providers threatened to 10x their premiums; etc.
It made entrepreneurship and freelancing a much harder choice for many (typically older-than-30) folks, especially since you risked getting fired & losing insurance if you worked alone or for a small-ish firm.
Some states had remedies for these issues -- either through a high-risk pool for people who were rejected in the individual market (though often these policies had coverage limits that made them not ideal), or through guaranteed-issue insurance in the small group market (i.e., if you and a partners start a business, insurance companies in your area who serve small groups HAVE to a) insure you, b) not charge you more than 10% the average rate, and c) renew your insurance when it's up for renewal, usually annually).
California in particular had both these in place pre-ACA.
Ask me how I know. :/
I’ve been managing a chronic condition for decades, and I’ve never had coverage denied because it was “preexisting.”
Maybe it was only if you were buying coverage outside a group it would be an issue, but I don’t know.
Regardless I many people who had preexisting conditions pre-ACA, and it never affected coverage.
For disability insurance, or life insurance, it’s probably a different story.