This might take a while.
On September 5, 2014, then head of the Riot Games Narrative team, Tom "TommyGnox" Abernathy, announced a plan to remove/separate Summoners, the Institute of War and the "League of Legends" from the game's story. This announcement was done through a Dev Blog instead of their long ignored lore discussion forums.
The announcement was met with about the same enthusiasm as a toothache removal if your dentist was Jason Voorhees, his "pretty" assistant was Leatherface and the only thing to kill the pain was a children's strength aspirin.
However, this plan was actually initialized two years earlier when Karma received her rework in June 2012. While fans of the character was most concerned about the loss of her... "fans," Karma's story was rewritten to remove all mention of the "League of Legends" and the Institute of War.
Other reworks had other distractions to keep people from noticing this change (Katarina lost all mention of her father and her story became that of "Generic assassin with breasts" and Soraka's lore rework changed Warwick, ruined Singed (which has not been fixed either), did not include the long-awaited "Battle Priestess" skin, etc.
So yeah, Riot's been trying to remove Summoners from the game for a long time. It's only now that we're fully aware of it.