What's the significance of "how they die" tags over every character?

I think the theme of the film is how precipitously the mighty can fall in the world being shown to us. You can end up an old man using a cane in a nursing home, estranged from your daughter or a toothless former boss dipping bread in frigging Welch's grape juice.

And you also can be shot at a relatively young age or disappear. Very few mobsters end up with a life that I would say is, in the aggregate, nearly as good as normal people who work in normal jobs. You may never have a big house but you also never have to look over your shoulder or serve time in prison or worry about your family being abducted. Look how John Gotti or Al Capone or Sam Giancana ended up.

The theme of the film is that organized crime is for suckers, working out well for only a very small percentage of those involved. The Henry Hill thing about being "movie stars with muscle" (if I remember the term correctly) is bullshit as he later discovered.