For a long time it was aftermarket HID kits. Installing those was a bit of a pain because of the separate ballasts and associated wiring. They were blinding because the projectors required a cut off and most were installed in halogen housings.

Now most people are installing aftermarket LED kits. For the most part those are less blinding. I upgraded our old Impreza with some but chose the lowest lumen numbers I could find and they are also engineered to emit light in the same locations and angles that the halogen ones did. I did some tests with the garage door and standing in front of it where a car would be and it's non-blinding for oncoming traffic.

There are even worse offenders. Some super cheap aftermarket kits seem to have the same horrific flashing as minimum viable Christmas lights.

Barely perceptible flashing yet somehow gratingly and distractingly so.