I agree. As a receiver, I laugh in the face of the very notion that am
obligated to do anything with a message other than as I will.
Which means you're also given the option to know what the signer's notion of
the signature's validity time is. And you're free to ignore it too.
If the signer doesn't provide it, the receiver has lost the choice to ignore it
or use it.
I can't say for sure that anyone is or isn't using it, but I would prefer not
to remove flexibility that's there now even if it was possibly a mistake for it
to be there in the first place (unless of course it's clearly harmful).
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