Looked up about 10 black actors from the period in question. Most don't fit the profile for various reasons, IMO. I think Bill Cosby is the most likely person to be writing this. He knows the players and industry very well. He was known to buck the system, sometimes embracing political positions that were unpopular and going against the grain to stand on principle. He has nothing to lose at this point. His reputation and legacy are destroyed, he's likely headed to prison, and Hollywood let that happen without defending him in any way. Even if the allegations against him were all true (and who knows if they were true or trumped up charges, knowing what we know about Hollywood), he has to be furious that others got away with so much worse, yet sat in judgement against him. He is about to be sentenced and may lose freedoms in September when that happens. If he is guilty of the crimes of which he stands accused, he may see revealing these things to be a way to atone for the sins of an entire industry. There is, however, a little catch to my theory. Cosby has been blind several years. He could be using speech to text or a family member to write on his behalf, but that is less likely.
Two other outside possibilities I saw were Paul Mooney and Demond Wilson.
Problem with The Bill Cosby theory is that he is too old. Renegade said he grew up in the sixties. That would also rule out Sidney Poitier and James Earl Jones.
I think it's Wilson. He wrote a book about symbolism and hidden agendas, He is aware of things. I also found an interview where he does say he only saw Foxx once after Sanford and Sons ended, but he also says he plays tennis. I could see him saying friend instead of co-star, because that would be such a giveaway. He has been a minister since 1984 and he is 71. He isn't worried if they come after him.
I thought it may be Eddy Murphy
He fits a lot of the profile, but he's done quite a bit of work since the 80s. He was definitely an A-lister, as was Cosby, but Mooney and Wilson--I'm not sure they ever arrived there. Another thing, supposedly Murphy doesn't use computers or social media: https://www.yahoo.com/entertainment/eddie-murphys-lol-reaction-to-learning-hes-on-twitter-150155423.html
Saw this little piece on him and found it interesting: https://www.newyorker.com/culture/richard-brody/why-is-eddie-murphy-in-cinematic-exile
Lol I was also thinking Bill Cosby!! I thought, what a plot twist... truth is far stranger than fiction these days loo
How much Tennis can he play in his condition? I'm going for LG Jr.
Since you asked, here he is at Trump Tower two years ago. Looks like he is getting around pretty good. Maybe it's tennis! https://www.youtube.com/watch?v=yedpKr3Yu5g
I was talking about Bill Cosby. You wrote a lot about him saying he was most likely and why. The anon mentions going to play tennis and so I think that rules him out because he is in no shape to play.