The number plates from the last few year's games are not in the legal or illegal appendices.
I assume they are not allowed for robot construction because of < R7 > c:
c. Only the VEX IQ components specifically designed for use in Robot construction are allowed. Using additional components outside their typical purpose is against the intent of the rule (i.e. please don’t try using VEX IQ apparel, team or event support materials, packaging, Field Elements, that are not listed in the Legal Parts Appendix or other non-robot products on a VEX IQ Challenge Robot)
I would also think that the plates could be used as a non functional decoration.
But what I THINK doesn't matter... How say you?