It's very sad news. I remember watching Diff'rent Strokes back in the 1980s when it was on in the 5.15 slot on ITV in the Central region (sandwiched between Children's ITV and the ITN News at 5.45), and I remember the characters so well. I seem to remember that Dana Plato died quite young as well.
Is Conrad Bain who played Philip Drummond still with us? He did look as if he was in his 60s when he appeared in the series nearly 30 years ago.