Tried creating the command, but it's not a thing.

Are you asking whether a vanilla convar exists called modularcar.insidedecayminutes? No, it does not. What is the reason you are trying that?

The plugin documentation mentions the vanilla convars such as modularcar.outsidedecayminutes simply for reference; the plugin does not create them.

oh okay.  Just checking, since it has an outside one.