First, I have to agree that Wikipedia's source Windor Public Library isn't that convincing. The only thing that Windor Public Library provided was this database entry:
The best evidence which could be provided here is a Death Certificate, but it seems you won't find that easily online.
Many users have been skeptical about his death and they have expressed their thoughts here, here and here.
I found an article published on 06 Jan 1993, by The Windsor Star, a newspaper, which seems seem to be pretty reputable, it has been published on "proquest.com". It has an abstract, can be cited and used in research.
If you can't access the article (due to country restriction or sign up/login) I have exported it to my GDrive here.
The article explicitly mentions that:
The 52-year-old Russian propaganda expert - who recently taught at the
University of Windsor - died Tuesday at Grace Hospital of a massive
heart attack following a life-long fight with alcoholism.
The article ended without mentioning an exact date of death:
Mr. Schuman moved to Windsor following his divorce in 1989 and began
teaching public relations at the University of Windsor two years
Just before Christmas, he was in Montreal for a brief visit with his
ex-wife Tess and their two children, 14-year-old Tanya and 10-year-old
He died two weeks later. The funeral is scheduled for Friday.
But we can estimate: So, 1989 + 2 years == 1991 or 1992 which seem pretty close to the time of death date by Window Public Library: Jan 7, 1993.
So, is that enough to prove that he died? Not necessarily, you have to remain skeptical; even if I provide you with a death certificate, it is well known that the guy has been a KBG spy with a fake name, which makes a faked death possible.