It doesn't seem well known - I can't find any many claims prior to the Summer of 2020.

I found one quote in passing from 2005 - note this in regards his working time frame from 1979 to 1995.

  You got an MBA at night a few years later. This should have put you right on track for a boss-type position at the bank. What happened?

  Well, one day, my boss called me in, and said they couldn’t promote a white male because there was too much attention on the fact that there was no diversity in senior management. She told me I didn’t have a future with the bank. And so I put my resumé out and went to Pacific Bell. A couple of years later, [Pacific Bell] told me exactly the same thing. And that’s when I started looking at cartooning as an option.

and

  You were placed on a series of doomed projects at Pacific Bell as punishment for mocking a boss’ memo in your cartoons. Did that actually work to your advantage by providing better grist for your cartoon mill?

  Well, it certainly made me angry, which is good. There’s a correlation between anger and humour. The angrier you are, the funnier you can be. You can drive things to the next level. But as far as material goes, I didn’t need any special bad projects to give me material – there was plenty.

I urge you to read the full interview - I suspect his lack of promotion may be more related to him as an individual than his race or gender.

https://www.itworldcanada.com/article/escape-from-platos-cub...

Given he is the only source for this claim, I dunno what you want or expect me to say.