All good answers above.
I didn't answer before because "essential" is much too strong a word, and with regard to competition, these days maximizing income in professional bodybuilding has nothing to do with maintaining health, other than in the literal sense of making it to the competitions.
I couldn't say that any given thing is essential: everything mentioned above is good, but quite likely for each named supplement there are pro bb'ers who do not take it.
Eating enough good food goes a long way for nutritional needs: anabolic steroids or GH do not I think create nutritional deficiencies. On concerns already mentioned of liver, blood lipids, blood pressure, and hematocrit, supplements don't do so much if anything for the last two. For ventricular hypertrophy and insulin resistance from GH and insulin use, supplements won't do anything for the first, and I doubt could do much on the second and don't have a basis for saying how much if any.
It's much more a question of managing drugs to keep harm no greater than tolerated than of powerfully-health-protecting nutritional supplements.