It seems to be a Roy Clarke thing.
Departing characters not mentioned immediately, typically this would need to be at the beginning of a the new series not to mention them.
Sometimes the departed character would not be mentioned until a couple of years later - in the case of Sid and Wally.
Billy Hardcastle and other characters just never mentioned again - usually characters that although missed, were presumably not as well loved as others - again Sid and Wally being the most obvious.
When a third man departed, despite both the actor still being alive, NEVER mentioned again, unless, in Foggy's case, there was a plan to bring them back.
Seymour's departure - very well engineered. Michael Aldridge had given plenty of notice, so a mentioned of Foggy was mentioned in his last FULL episode, and Michael Aldridge appeared for five minutes in the first episode of a new series, in which Foggy re-appeared (though the characters don't meet), and Seymour is even mentioned by Edie towards the end of the episode.
Obviously a lot of effort put into Bill Owen's character.
Nora's two departures mentioned - had to mention her as a she was an important part of the show, though quite discreet mentions.
I think Wally was mentioned following his death the most. He was mentioned after a year or so it Crums - then I don't think he was mentioned again until the 90s. Sid was mentioned in Uncle of the Bride, not by name, and I don't think he was mentioned until the mid 90s, by name, and then we saw his photograph in the Compo trilogy.
Edie seen in pictures, can't recall if or how she was mentioned verbally.
Don't think Wesley was mentioned as such, though I'm certain others can help.
Usually if a minor character is 'replaced', such as Sid or Wesley, they tried to leave an episode without the replacement, then set about introducing the character, in these cases Milburn and Entwistle.