Millions of white people gaslit themselves into believing black people are biologically inferior, with entire disciplines like phrenology being made up out of thin air.
Huge conspiracies are quite possible, if they take the right shape. Hide the alleged facts behind experts that everyone chooses to trust for no reason other than that they want it to be true. Make sure your claims are unfalsifiable because of time, expertise, or deceptive vagueness. Have "eye witnesses" make vague claims or none at all. Get people invested in propagating the lie by tying it to community and morality and social fabric and profit so people struggle to let go even if they disbelieve part of it.
The holocaust does not fit this pattern, but there is plenty of "common knowledge" that does.