The league-v3 getAllLeaguesForSummoner endpoint will be deprecated on Wednesday, January 10th, 2018. Please use getLeagueById as a replacement for this endpoint. Though we plan to deprecate the endpoint around noon Pacific time, we suggest having a solution in place at least 12 hours in advance that is ready to swap over as soon as the endpoint is removed.
For the most up-to-date information regarding changes to the Riot Games API, we recommend following our Twitter and checking for new forum announcements.