It's not a stock app in iOS, but it was preinstalled on new phones...similar to how Pages, Numbers, Keynote, iMovie, and GarageBand are sometimes preinstalled. Clips was preinstalled on my iPhone XS when I got it in 2018.
I didn't set up my iPhone 13 and iPhone 16 as new out of the box when I got those, so I can't confirm if it's no longer preinstalled...but OP may be onto something. It has been a while since Apple has updated that app, and it still doesn't have a dark icon. Most of Apple's other apps have been updated to support iOS 18's new light/dark app icon feature. Not sure if that means if they're phasing it out, but certainly looks like they aren't giving it much attention anymore.