Are there any manga that used to be extremely popular but then something bad happened (either in the manga or in it's surroundings) so that after it was made public it was kinda collectivly decided to never talk about the manga ever again?
Besides the already mentioned Usagi Drop there's also Gal Cleaning and its many sexual assault focused .x chapters after it got axed if I remember correctly - don't think many people talk about it these days.
Monster Musume seems forgotten by all but one of my obsessive friends. No drama, it's just that the author wrote himself into a hole and can only write status quo stories now.
Possibly. It's hard to say you don't talk about something without talking about it. And it's hard to say if we're actively forgetting something or if we just forgot something, and if we remember either, it isn't forgotten anymore.
Moe Harukawa had another manga besides of the Touhou series she was assigned to (Forbidden Scrollery), and it ended after 2 or 3 chapters, with no explanation and with Harukawa also disappearing.