Technically since Obamacare supposedly got everybody covered, there are currently no pre-existing conditions.
The issue would be people born after the Obamacare repeal who decline to buy health insurance and then develop a condition.
I think an idea talked about by Republicans was high-risk pools or something like that that would have higher premiums.
Not exactly. Pre-ACA, being insured for a condition was no guarantee that you would remain covered if you changed insurance plans, or even if your insurance company chose to drop you at the end of your annual contract.
As a practical matter if you had group insurance through your employer you wouldn't have a problem, but the individual market was brutal.
I have personal experience of being unable to get insurance, at any price, when COBRA coverage expired, despite having been continuously covered for over 30 years.
You're right in that with the ACA insurance companies don't even ask about your medical history when writing policies any longer, but I assure you that if the ACA is repealed people with medical problems in their past will be right back where they started.