OK I have done more homework in to what Nicola Charles said. So the previous six names I mentioned this was their last episode.....
Marlene: Left in October 1997 for a three month cruise and never returned. Died in real life January 2018.
Helen: Also left in October 1997 and died in real life in 1999 after two decades of ill health.
Cheryl: Last appeared September 1996 and still alive.
Angie: Ended her first run in 1997, last appearance was 2017. Still alive.
Madge: Last appearance 2015 as a "vision". Still alive.
Pam: Has a brief return in 1996 so wasn't one of the big characters by that point. Last on the show 2016 when her characters husband Doug died.
So I wouldn't say it's Madge, Angie or Pam as they have been brought back since the 90s so the show hasn't washed their hands of them. Cheryl Stark was played by Caroline Gilmer who was ill so replaced by Colette Mann who now plays Shiela and then she took back over to be killed off.
Could it be Cheryl was killed off because of it? Caroline Gilmer said made out she left for other acting opportunities but I haven't seen her in anything since Neighbours and her partnership with Lou Carpenter was very popular.....
Helen if she had ill health for twenty years I think if it was her they might have used that as a reason for writing her out sooner than the year after the incident in 1997.
Marlene played by Moya O'Sullivan could be the one......
Moya O'Sullivan was an Australian actress who played Marlene Kratz from 1994 to 1997. Date of birth - June 8, 1926, Australia Height - Date of death - January 16, 2018, Bondi Junction, Sydney, Australia https://en.wikipedia.org/wiki/Moya_O%27Sullivan
neighbours-soap-opera.fandom.com
She would have been seventy in 1996 and it's the sort of comment a older person would say more likely by not realising how offensive they have been as a generational thing. How silly her leaving storyline was which seemed abrupt and that she never returned looks like it a rushed decision forced on them. Nicola not naming a name makes me think it's someone deceased who can't defend themselves.