I don't think oversaturation is the right term.
Gen 4 in general and Sinnoh in particular went out of their way to include large numbers of legendary Pokemon. Sinnoh (Platinum) had 14 new ones (not counting that stupid retroactive mythical classification of content gamefreak locked behind an action replay), nearly double Hoenn's and more than Kanto and Johto combined, and 20 total counting the Regis and Bird trio you can later capture. Unova couldn't top Sinnoh but they made the main legendaries more story-relevant and still had a good amount to stumble on.
Gen 6 is where legendary pokemon fell off. Yeah, you could capture every legendary Pokemon and sundry in ORAS - as part of a roaming mechanic, and those legendaries had no context for why they were there or what they do or anything that really made them interesting. ORAS could at least fall back on the original Hoenn legendary fluff, but Kalos was even worse; it has only three legendaries, only two of which were actually cool at the time since Zygarde is essentially the biggest piece of wasted potential in Pokemon media, and three event legendaries that took so long to come out you'd be forgiven for thinking they were Gen VII mons (which I originally did because Alola's legendaries are also boring as sin).
It's not so much an oversaturation of legendaries as it is an ability to make interesting new ones. The old legendaries were cool because of the fluff around them and the ways you would use to have to catch them as much as their designs and stats, because Pokemon used to be about actually exploring a world. Gen 6 and on legendaries mostly suck because Gamefreak started making the games much more linear and streamlined, removing a good bit of the mystique around legendaries in the process, and rely on the nostalgia old legendaries generate to make up the difference.
