Insurance companies haven't been allowed to deny coverage for preexisting conditions since 2014, due to Obamacare. They can't even raise premiums. The only variables are age, sex, smoking, and where you live.
This applies both to employer plans and the ACA plans on healthcare.gov, where you can get a quote without giving them any health data at all.
But, you can get absolutely destroyed on life insurance and, just as critically, long-term care insurance. Genetic discrimination is perfectly legal in those domains.
Generally it runs aground with things like the ADA because you can't just "accidentally" find out the person has a covered condition, you're just never allowed to ask or require it be told. Exceptions for something like an airline pilot on matters related to the job like sight notwithstanding. This is why you always see things like "can lift up to 40 lbs" type requirements instead.
This applies both to employer plans and the ACA plans on healthcare.gov, where you can get a quote without giving them any health data at all.