The only Gen that doesn't have a way to get easy shinies is Gen III. Gen II and Gen IV-VIII all have different methods to greatly lower shiny chance, and I believe Gen II, the gen the mechanic was introduced, is second lowest of all, with there being a glitch-free reliable method to have a 1/64 chance to get a Shiny of almost any breedable Pokemon just by having your free Shiny Gyarados be Male(lowest is the Cute Charm Glitch present in all Gen IV games, which can potentially create a 1/10 shiny encounter rate with any wild pokemon species that is capable of being either gender).
So in my own opinion, it is less that they became more common and more that the fanbase overall became more obsessed with grinding for them over time.