I had a dwarf -- a miller by trade, I remember well -- who was killed in an ambush. He was knocked into a pool, where he drowned; the body was never recovered. I made a slab to commemorate him and placed it on the spot where he last stood in life. His son was quite upset for a time, and as 'time heals all wounds,' he eventually grew up and took up a pick in the civil service of Counselledceilings.
Years passed.
The seasons came and went, and the caravans with them. Another ambush, a siege, groundbreaking on the new arena and the training-up of a few Master Lashers, armed with the whips and scourges of the same ambush that killed the miller Risen Ikudrimtar.
After the second siege came, as I was cleaning up the mess and generally straightening out industries that had been disrupted by the lockdown, I noticed a peasant corpse in the same pool, on the same tile, where Risen passed away. Yet when I checked the corpse, I found that it was not an unskilled mook who just happened upon the same fate as my late miller, but the body of Risen himself! I checked his son's Relationships screen: Risen Ikudrimtar, no listed profession, named in white.
To the brave soul of Risen the miller: Armok remembers you, even as your comrades and family forget.
[NOTES]
During the last siege Risen's slab got toppled and I set it up again after the siege. One of those two events may have triggered the 'reset' on his data. DFHack's 'deathcause' shows his death as more recently than when it actually happened, and says "cause: memorialized."
I know it's also possible that the game "forgets" units after a time to cut back on savegame size and processing requirements. Toady puts a lot of culling features like that in for the sake of optimization; if that's what it is, I find an interesting parallel with how people IRL sort of offload memories of people they haven't seen in a while so they can focus on more recent and pressing matters.