The changes in patch V126.96.36.199 were likely made to prevent Sona from abusing the item.
Its eventual removal was most likely due to it becoming a source of near-infinite sustain for champions with low ability cooldowns (Udyr in particular).
Numerous times was Innervating Locket rumored to be reintroduced (at GamesCom 2011, before Season 2012 started) but it never did and was replaced by Locket of the Iron Solari.
On Nexus Blitz's release, streamer Hashinshin requested via Twitter that Innervating Locket be reintroduced (his post must hit 4444 retweets to be added in). It was included when Nexus Blitz was released in V8.24.
Unique Passive - Eternity: Restore mana equal to 15% of damage taken from champions. Restore health equal to 20% of mana spent, up to 25 health per cast, while toggle abilities can heal for up to 25 per second.
Unique Passive: After using an ability, restore 8% of missing health and 3% of missing mana over 3 seconds.
Caption:"Cast down the Noxians" – Bobdyr, Temple Guardsman.