That I know, but those labels can't be guaranteed to to get left stuck
in place forever. But, now that I think about it while I'm keying this
in, a "two pin" receptical might encourage someone to use a "2 to 3"
adaptor and connect its ground lead to the cover plate retaining screw,
with the mistaken assumption that it would provide the ground. Sometimes
you just can't idiot proof stuff enough, huh?

Will, the GFCI gives protection regardless of whether the frame of the
appliance is grounded.

IOW: even if you "think" the frame is grounded but it isn't the GFCI will
still protect you.

The label is to let you know what's going on if something doesn't "look
right." Say, for example, you plug in one of those "testers" and it says
"MISSING GROUND." If you have lost the label you might be concerned and do
some checking. If you see the label you say, "I knew that!"

Likewise with downstream GFCI outlets: if the label is missing and you
trip the GFCI you might make an unnecessary trip to the CB box rather than
checking for a tripped GFCI in the same or nearby room.

These labels don't warn of a Hazard but only tell you that what's really
there isn't quite like you think it is.