-Keith David was not announced at D23 as a returning voiceI know the consensus is that he won’t be included, but what’s your source for the latter claim?
-Keith David is a prolific voice actor who is still active (recurring role as the President on Rick and Morty)
-Keith David has worked with Disney recently (Ducktales, and I won’t spoil his cameo in the finale)
In short it is highly unlikely that Keith David would decline an invite to voice the shadow man if Disney asked and it’s highly unlikely Disney would hire him and not announce him along with the returning voices.