Is 40 year old unopened seagrams VO Canadian whiskey safe to drink?

No. Well, any alcoholic beverage is dangerous, if you drink enough of it. But 40 proof is not very strong. Most liquors -- vodka, gin, rum, whiskey, etc. -- are 70 or 80 proof (35% to 40% alcohol). Some are even higher. Everclear is 190 proof (in some places), and Bacardi 151 is 151 proof. Forty proof is relatively low.

Proof standards are slightly different in different countries. In the US, 40 proof is 20% alcohol; in the UK it would be a bit under 23%. In either case it's higher than can be obtained by straight fermentation (for example, wines or beers), but lower than distilled beverages. A 40 proof beverage is most likely a "fortified wine", such as port, sherry, or vermouth.