I'm pretty sure you can still get the summons even if they came from the past banner. (eg. Lily, Naveed and Cerberus) But i doubt you can still get L.Elly except if Halloween banner will show up next year (if she's included again).
For better information you should check the First Encounters from the Summon tab. Every adventure that are on it are still available to be summon yet the rate are lower unlike when they're on their respective announced banner.
As for the Loyalty's Requiem Raid since they're not part of banner I think you can't get them. Except if Cygames would pull the same event over again ofcourse with the same rewards.