I would like to repurposes a perfectly usable legacy copper wire phone company plastic box, to contain a ground plate and a lightning arrestor gas discharge tube.

Is that box truly mine to use? There are NO functioning legacy phone lines in the home, and I doubt anywhere else.

This box would simplify my installation as it already is mounted, already has access to the service ground rod, etc.

Do it

Even if you revert to a "land line" from your local telco, they will not re-use that box. Instead they will install a new, larger box to house their fiber termination equipment. In your place I'd go ahead and use it for whatever purpose you want.

The good news is that there should be an electrical ground and lightning arrestors in the box already.

Other news is that the box is there to protect your house from lightning surges coming through the copper wires and (hopefully) preventing your house from being burned to the ground.

The bad news is the government regulates what items can be attached to said box and/or its contents.

I used my outdoor phone box for trash after I salvaged the brass nuts inside.

Are there any Telco wires coming into the box ?

I ripped mine out and the telco wires that went to it are buried far under the ground now. Easy for me to get rid of since all wiring is underground here. If it were overhead I'd likely have let it be.

As I understand it, building codes no longer require a traditional phone wiring like they once did.

At least in most situations. I think if you haves an elevator or some such thing the requirement may still exist in some places.