Edit 1: users report, that as of May 2021 DLC might also count. So your milage my vary. The general How-To still stands though. Happy searching!
For this you need 200 different items of apparel, colors count!
If you find a black gas mask and a green gas mask, that is two (2) items right there!
I unlocked this after picking up a "Short pleated skirt, green/black+" which was my 4th Short pleated skirt.
Apparel items can be found in backpacks, tool boxes (the colorful ones, not the green ammo crates) and on dead bodies you find along the way.
You need to be patient with this one, as the spawn is so random, that i got at least a dozen green gas masks and for some reason brown boots.
IMPORTANT NOTE - DLC Apparel does NOT count towards this achievement!
DLC apparel has an arrow pointing down besides them on the profile screen.