Even if The Simpsons wouldn’t have been able to afford more principal cast members beyond its core group back in the earliest seasons, the show did have Black actors Ron Taylor and Daryl Coley voicing and singing for Bleeding Gums Murphy for his Season 1 appearance and beyond.
All that said, Harry Shearer is clearly not reading the room very well by speaking out in mild opposition to the idea that white actors shouldn’t play PoC in animated series.
Diversity has obviously become more of an imperative across Hollywood as of late, with modern-day blackface comedy having only recently been taken to task, so even though the world of animation isn’t the most obvious source of cultural misappropriation, there have clearly been lots of questionable situations on that front throughout the medium’s history.