What do you say to someone like me, I cannot remember the forms, and frankly lost interest in doing them. I prefer to avoid jumping kicks and rarely sport spar anymore. Am I no longer a blackbelt? That being said, I kick with lots power and flexibility, remain in good shape, and incorporate pieces or forms to something I find useful.
You still earned your black belt at whatever time you earned it. (Presumably at some point, you did know your forms and did do sparring.) That can't be taken away from you, even you don't practice or remember that stuff much anymore. It's like getting a college degree. But we wouldn't award a black belt to someone who didn't remember their forms and didn't do sparring, because they haven't earned it.