Chadwick Boseman's full commencement address at Howard University in 2018
was 36 minutes long. Kamala Harris retweeted this 8-minute segment
which is all about Chadwick was hired and fired on All My Children
for the role of Reggie Porter (the character later adopted by Jackson Montgomery).
Almost all of these 8 minutes are about racial representation on TV/film, using AMC as an example.
I won't try to sum up what he says -- just listen, he says it so eloquently.
Note that he mentions the lack of specificity.
fyi, he mentions that he hopes that his questions to the producers about stereotyping helped to change the way the character would be written for the Black actor who replaced him. (He doesn't give the actor's name in this speech, but we know that the Black actor who replaced him in the role was Michael B. Jordan).
Here is the entire commencement speech, which gives context to why he spoke about his AMC experience:
If you watch the whole youtube, there is more. he said he was sort of blackballed for a while because AMC had put out the word that he was "difficult".