Adding a new person as dead that is less than 100 years old should require prof of death.
There is inconsistency with this point. When adding an individual (say a spouse) via the source linker a reason statement is required (albeit one can just enter a dot (.) in the field), but nothing is required when adding an individual from the Details page (of a parent, spouse, etc.), even if the birth year is quite recent.
It does seem reasonable for a birth date of less than 110 years old to trigger a warning message, but what if the birth field is left completely blank? Would any warning then be based on when, say, the parent lived - recently, or perhaps hundreds of years ago? Maybe the coding that would produce a "proof of death" is just not so straightforward to write.
In addition to what Paul said, there are also typos to consider: it's Very Easy to get into the habit of typing whichever century you used most recently.
And, more importantly, exactly what would constitute "proof"? Recent events very often have nothing "official" available, by any definition, because of misguided privacy rules (such as the decision to give identity thieves free rein by not publicizing the SSDI for recent deaths). In any case, you would not be able to require the attachment of a source, because nobody can attach anything to a nonexistent profile, and, as Paul implies, requiring a reason statement is just a useless annoyance. (When the Source Linker-based process requires one [usually for someone whose death information I Just Finished Entering], I generally use an exclamation mark [in place of the expletives that I actually want to use].)