I remember back in the late 70s early 80s my daily driver had radials and after maybe a week there were some flat spots when parked, but they rolled out after a couple miles of driving. I remember the sales people advising that would happen.
My daily driver now stays parked for extended periods and no flat spots are present on my Michelins. I have never found flat spots to be a problem on today's tires.
Back in the day, I had a new '70 challenger with the "new technology" fiberglass belted tires. They were always flat spotted after a couple days being parked, but rolled out after a couple of miles.
I park my '70 Challenger with BF Goodrich Radial TAs all winter for several years now and never had a flat spot problem. I'm not a fan of the TAs, but they haven't flat spotted.