I second (or third, fourth etc ) to follow the manufacturer’s recommendation. But break in is real and a fairly big deal for some cars. It’s not correct to say it’s a thing of the past.
For a BMW M2:
“Up to 1,200 miles/2,000 km
Drive at varying engine and road speeds, but do not exceed 5,500 rpm and 106 mph/170 km/h.
Avoid full load or kickdown under all circumstances.
At 1,200 miles/2,000 km
Have drive-in checkup maintenance performed.
From 1,200 miles/2,000 km to 3,100 miles/5,000 km
The engine and road speed can gradually be increased to a constant speed of 137 mph/220 km/h.
Use the maximum speed of 155 mph/250 km/h only briefly, for instance when passing.”
I get a chuckle out of that last line. But anyway note you’ve actually got to get a maintenance check at 1,200 miles. And from I’ve heard directly from techs that’s not to be skipped: they replace special break-in fluids and there will be metal shavings from the break-in especially in the car’s limited slip differential. That’s perhaps a slightly extreme car, but not a very extreme car.
Owners of cars in that general category debate on the web whether you should do things like try to fairly heavily load the car’s engine, though not flooring it as BMW prohibits. Some say doing that will materially help the rings seat properly as opposed to driving very light footed in the break in period. But as you can see BMW doesn’t recommend anything beyond varying speed, limiting speed/rpm, and no flooring/kick down (latter means pushing the gas pedal beyond the first detente on the ‘floor’).