I'm not sure how user IDs are assigned, exactly, but I'd suspect that they're not set in stone, given that they're only six digits. If that's the case, then such a function wouldn't serve much purpose, as it'd stop working once the person is assigned a new ID.
EDIT: Actually, I can confirm that they're not set in stone. I made a thread a week ago where my assigned ID was 31b1a, and I made another one just now with the ID of 73e186. So yeah, wouldn't work.
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