<R19> refers only to the Identification Plates found in the VRC License Plate Kit (276-3938 ) and that teams receive in their Welcome Kits. The old style of pre-printed license plates cannot be used for identification to satisfy <R19>.
They could be used as a non-functional decoration, provided that they do not pose a risk of confusing inspectors or referees. That is, you should not use old license plates with a different team number than your current one.
Regarding non-functional decorations, rule R7d part ii includes the following statement:
Any guards or decals must be backed by legal materials that provide the same functionality. i.e. If your robot has a giant decal that prevents Scoring Objects from falling out of the robot, the decal must be backed by VEX material that also prevents the Scoring Objects from falling out.
Your statement of "the robot has the same functionality with or without them" would seem to fulfill this requirement (again, provided that this is the conclusion reached by an inspector / referee).
That being said - since the old license plates would satisfy the requirements of <R7f>, quoted below, they could even be used as a functional component, provided that no other rules are violated in the process.
<R7> Robots are allowed the following additional “non-VEX” components:
f. Non shattering plastic from the following list; polycarbonate, acetel monopolymer (Delrin), acetal copolymer (Acetron GP), POM (acetal), ABS, PEEK, PET, HDPE, LDPE, Nylon (all grades), Polypropylene, FEP; as cut from a single 12" x 24" sheet up to 0.070" thick.