I grew up in an all-white neighborhood In the 1950s, and didn’t even meet any Black people until I was a teenager. I didn’t see Black people in movies or TV, and the only black music in my house was my father’s Louis Armstrong records. my eight-year-old friends used the word “spaz” all the time, and we all knew it was short for spastic. My guess is that young black boys were using it at that time about as often, and in the same way, as young white boys were: Because it was a cruel taunt to use against each other and anyone else who was different.
Spaz may be part of AAVE just as words like “table”, “chair” and “six” are part of AAVE. but no person who speaks English is appropriating anything from anyone when they use it.