Sorry for your loss.
A necropsy would be a good ideal...
https://www.backyardchickens.com/threads/how-to-send-a-bird-for-a-necropsy-pictures.799747/
https://www.aphis.usda.gov/animal_health/nahln/downloads/all_nahln_lab_list.pdf
I have to wonder why are you treating multiple birds fro bumble foot? Should we review your set up and see if there are changes to be made that might help avoid this in the future... for example roost height or landing surface?
It could have been kidney issues including gout or failure (causing sudden death) if he has been eating "layer" feed his whole life... as too much calcium fed long term to birds not in lay *can* (doesn't mean will) happen. It is often in birds that may be genetically predisposed or have been fed excess treats. Sometimes even the fancy non pelleted feed can allow them to pick out just the goody stuff.
Also, how bad the infection was could have had an impact. If it was just a scab maybe no big deal... but if was all fill of puss and such... maybe the infection was effecting other parts of the body as well.
He must have been a pretty good boy to make it 4 years! I would love to see a pic in his honor if you have the time and desire to share.
IMO... there is NO such thing as SDS... there is
always a reason and that's just what people say who don't know why their bird died.
Heart attack?? I would wonder about blood clot causing a stroke type situation... kinda like they do with human patients after surgery. I am NO expert... and just here to share and learn like most other people. But my mind SPINS as many possibilities as it can possibly come with in the search for truth. I NEED to know the truth.
ETA: I see the SDS poster also states it IS underlying issues.
