Cuck porn started becoming common in around 2015 I'd say
Having actually been active online since the mid 90's, and a long time worshiper of Pervos, God of Smut, there's been a lot of changes in what porn has been popular every few years.
The late 90's to early 2000's had far more parody porn, professional porn studios making most of what was out there, and most stuff was pretty artificial and tame by todays standards, while the niche but popular enough to be common stuff was anal, gangbangs, watersports and facials. It was a simpler time.
Then through the mid to late 2000's there was a surge in weirder shit due to the internet allowing fetishists to get together.
Bukkake, feet, tentacles, furries, hentai, really big natural titties and their sisters, the Phat Assed White Girls, along with trap models starting to become a popular taboo, meant porn was becoming far more diverse and catered to more unusual tastes more readily, while still being mostlyprofessionally made.
However, this was also the era of the celebrity sex tape, with Paris Hilton and Kim Kardashian building entire media empires, coming off the backs of letting dudes film themselves cumming over their backs.
In to the 2010's and we'd started to see trans porn, cam girls, a sudden deluge of amateur video, mainstreaming of streaming sites like pornhub, previously looked down upon kinks like analingus becoming not just accepted, but weirdly desirable and entering pop culture, interacial porn and the objectification of black guys and 'Big Black Cocks', alongside the fake "incest" porn.
Big studios began falling to the wayside, and with everyone being online, everyone having the ability to film themselves, and the ubiquity of the idea of sex tapes due to the celebrity ones previously, porn creation and experimentation simply exploded all over themselves and each other.
As I said though, cuck porn really started to hit it big in 2015ish, about the same time Overwatch made cgi animated porn mainstream.
I'd say it's really died down recently though, and become subsumed into the more aggressive side of the interracial stuff that's far more common, as well as soemwhat into trans porn, to a lesser extent, which is also super common now.
