EE: I think some of the Slaters have had no lasting impact, even when Kat was around. She never really talked about Lynne and Little Mo, and Billy never mentions his time with Little Mo, either. Meanwhile, Trevor, Garry, Minty and Dawn all had big stories/were featured prominently, but have fizzled out.
I think Pat has also become a footnote. The characters that she impacted the most - Janine, David - have gone. The Butchers has a whole have become quite irrelevant; Ricky was a major character for years, but left no lasting impression; no-one remembers Natalie.
Neighbours: Julie Martin is never mentioned. Paul, Daniel and Lucy have all talked about Scott and Charlene, Jim and Helen (we've even had flashbacks), but not once does anyone remember Julie - or any of the Martins, either.
Also, Robert Robinson seems to be a forgotten character, despite him blowing up several major characters, and trying to kill Paul. The guy is in prison, but you'd never know he ever existed now.
Home and Away: Most of the major characters of the past haven't left any legacy on the show. New viewers will never know about Selina, Saul Bennett, Stephan, Carly, Bobby, Duncan, Jesse, Hayley, Noah, Kane... Or any of the Sutherlands, Holdens (despite Jack being a popular character at the time), or that Irene has biological children.
I think once Kyle leaves, the Braxtons will go the same way, too.
ED: Lady Tara, Kathy Glover, Rachel Hughes, all of the Sugdens that aren't Jack and Sarah, Scott Windsor.