Proposition 21
Proposition 21 was a roleplay thread posted on /tg/ on May 07 2013. It took the form of a political ad urging readers to vote yes on a fictional "Proposition 21" which would legally enshrine Mutant segregation and the criminalization of so called "dysgenic unions." Like most things /tg/ likes, the fa/tg/uys immediately added Catgirls to the mix. Eventually /tg/ posited a future after rampant genetic engineering and a massive nuclear war, mankind had split into two distinct races. "Norms" so called because they have not been altered by the Radiation or renegade Gene-hacking retrovirus and the Mutants who had been. /tg/ also created a third group called "Clades" to justify the existence of Catgirls and other Kemonomimi-type characters in this universe. Of course both Clades and Mutants suffer intense discrimination and persecution from their Norm neighbors.
Eventually, Proposition 21 would spawn two divergent universes. While quite different in tone, both maintain the central conceit of a segment of humanity who were Mutated and now suffer discrimination because of it. Clade Quest and Bloodhound Quest
Also like most things /tg/ likes it is pretty much a direct rip of 2000AD, specifically the long-running series Strontium Dog