This is actually one of the biggest things in league that still annoys me to this day. For a world that is actually incredibly cohesive and alive, with all these champions interacting with each other, Riot never bothers to highlight these interactions in-game with old champions. Sure, the new champions get interactions, but what about the old ones? Is Lucian just going to keep yelling "For Senna!" every time he ults even though she's with him again?
If you want a better example, lets head over to Riots multiple comic series they've released.
Sylas is a mage that seeks to release Demacia from its oppressive ways of punishing mages for whom they are. Sylas has a lot of interactions between many of the highest ranking and well known people in Demacia, between Jarvan, Garen, and Lux. Sylas got voicelines for most of these characters. However, Lux, Jarvan, and Garen still seem to sit on the rift oblivious to what happened between them and Sylas. When you release a new character, you have to accept that the characters around them also evolve, for better or worse and that should be reflected with mannerisms within the game. As it stands right now, only the characters getting released are really showing any knowledge of events that have occured within the universe, and it does pull away from the immersion. Its a very glaring issue, and it should be something that is addressed sometime soon up the road.