As mentioned to mr_mischief, I was thinking that (pseudocode):

if today >= die_date
    if 'if' is present
        if 'if' returns true
            die reason
        else
            warn reason  # they won't forget it now!
        endif
    else
        die reason       # automatically die if no condition
    endif
endif
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Thus, they get deprecation warnings even after the 'die' date, even if no code triggers the deprecation. I'm not sure if this is a good idea or not.

I'm also thinking about aliases to the deprecation name: deprecation_XXX and deprecation_TODO. Programmers often embed XXX and TODO in their code and grep for them regularly. This will make it easier to find.