How do you handle sims dying off lot?

You can use cheats to get the tumbstone (or urn) from the Buy catalog. Probably "testingcheatsenabled true" (or was that for ts3, LOL, I rarely use that code). There would of course be no name or ghost.

I use MCCC and have the sims die on lot setting enabled. So most times I'm able to get their tombstone even if I wasn't playing them at the time.

Back when i played with aging on for all sims, i didn't know my sim self's father passed away until i got a pop up to say his spirit is fading away while playing as my sim self. I switched over to my sim self's parents apartment and his father had already died and his mother was a few days from passing away next. I added her to my sim self's household and she died the next day.

A few sim years had passed on and their ghosts were sent to the netherworld but from old screenshots i was able to re-create both of them in create a sim and "bring them back to life"

You could always bring him back to life and kill him again, it's a lot of work but you can use cheats to up you cooking and gardening, or you can probably find ambrosia in the gallery.

If another sim is still alive in the house that the sim died in, your sim can visit the house and the urn should be there (usually right by the front door) and you can put it in the sim's inventory. If there are no living sims in that house though, you can't get the urn or there just isn't one.

What I did in my legacy is when I moved my next generation young adult out leaving elderly parents, I made like a young adult home nurse in CAS and moved that sim in with the parents so someone would still be alive to be able to get the parents urns when they died.

I am so very sorry this happened to you. Back in Sims3, I found it rather odd that Joseph Cantrell, Sr., showed up at Erik's house out-of-the-blue. I don't quite recall now, how I handled it. I know that very shortly after the spur of the moment visit, Erik got the news his father had passed away. It was very upsetting. Because I wasn't playing his parent's household there was no urn or even a tombstone. Joseph was just nowhere.

In Sims 4, in a past Main Save, Erik's father passed away on his way home from Erik's twin daughter's birthday party. The girls had just become Teens. There was no urn, or tombstone, either. My Sims were a wreck. I believe, if memory serves that I attempted to get him back so I could have him pass away on his son's lot. When that failed, I rolled back a save. At that time I was using the Mortem mod by SimRealist. Once I had the urn, Erik was able to have a proper funeral for his Dad so that the family could come to grips with the loss of their patriarch.

This is a useful mod I have. It was last updated over a year ago, but seems to still be working.

https://modthesims.info/d/652443/urn-binding-and-summoning.html

You can buy urns in the Buy mode catalogue, and 'bind' ghosts to them.