Usually when a missing person is declared dead without any physical remains, cause of death will read "in absentia," which means that there was reasonable evidence to presume them dead even though there is a complete lack of physical evidence to determine the information usually needed for a death certificate (location, date, COD, etc).
In the U.S., death certificates are public domain unless the deceased is a minor. The difference is "general" death certificates versus full death certificates. General death certificates are public domain, whereas full death certificates may be restricted to immediate family, law enforcement, and government. The difference between the two is in the documentation of cause of death. A general death certificate will specify accidental, homicide, suicide, in absentia, or natural death (illness, health conditions, disabilities, disorders, etc). A full death certificate will have details of the natural death, such as the specific injury or illness that led to demise. If a person was under age 18 when he or she died, the legal guardian can request that the death certificate not become public record. In these cases, it will only ever be able to be accessed by immediate family.
The thing is, those are the national regulations put in place by the U.S. But,
each state is allowed to expand upon them and make additional restrictions. In Missouri (that is, deaths that occurred in Missouri- that is how they are regulated, based on the state that the person died in), the release of certificates are restricted to authorized persons: guardian, spouse, child, descendant (genealogical research), legal representative, or professional genealogical researcher.
The Missouri guideline for death certificates becoming public records is 50 years. So, if you wanted the death certificate of someone you are not related to who died less than 50 years ago, you would need to find a business licensed genealogical researcher to work on your behalf. I believe that it is Canada that has implemented the 72 year rule nationally. Delaware also has a really long waiting period... I think that theirs is about that, as well. Texas only has a 25 year restriction. So, you can see how it is going to vary greatly depending on what state you are working with. U.S. census data also has a 72 year restriction, is that maybe what you were thinking of?
Sorry this got kind of long, but I hope it helped!