The confirmation came from the Steam support staff earlier this month when Resetera forum user delete12345 asked Steam support if he can put his Steam library in...
If you cannot pass on your ownership rights to your purchased games to your children, then you cannot pass on your copyright either, I guess?
IANAL, but I feel like if the heirs to an estate cared enough about the deceased's Steam account enough to get the court involved, Steam wouldn't have a leg to stand on. But that's probably what it would take to get them to do the right thing.