I got the classic SP notification to visit Abby Sample, who didn't have much longer to live.

The problem is that Abby's dead. And she didn't die recently. She died sim-weeks ago (on a 100-day lifespan), and her widower has had time to settle down with a new boyfriend.

Does anyone knows what might cause this SP notification to go visit a long-dead sim?

Could you check a list of all sims town, through one of the filter prompts from StoryProgression or MasterController? Either by choosing no filter, or surnames starting with S.
I'm not sure why it would inform you if you weren't friends with that sim, but it is possible there is a townie walking around whose name generation got particularly uncreative and sampled directly from an in-world sim.

My only other thoughts would be some kind of travel desync or other way the game might still be holding onto an instance of that sim somewhere. Fun serendipity for haunting their widow though!

I did check to see if there was a living sim in the world by that name. It's actually a former member of my active household who moved out two generations ago, so the older household members were close to her. The default name search, which I believe only searches for living sims, did not return any results.

My active has traveled several times to different EA neighborhoods using Traveler (I love doing that). That sim was dead the last time she traveled but might have been alive the time before that.