The Great Big Book of Horrible Things is a list of the 100 worst man-made events in history, many of which fit your definition of moral catastrophe.
Practices (rather than events) that might fit your definition include
slavery, in its many forms
female genital mutilation
Apartheid, and racial segregation in other countries such as the USA and Australia
corporal punishment (especially of minors, by parents, in schools etc)
rape in marriages /marital rape
“marry your rapist” laws
with-hunt (though I am not aware of the # of affected individuals)
I also want to note that the things I have added and many others added are still ongoing. It would be naive to say that these are only moral catastrophes of the past.
A few more controversial moral catastrophes:
religion (arguably counterfactually responsible for at least a few wars and a few really unhealthy cultural traditions, though it is hard to say whether the counterfactual would have been much better)
no available education for everyone—under certain moral philosophy (I am thinking of Mill, in particular), the access to academic thought and literary texts, critical thinking is of critical importance. Billions were barred from it.
Male genital mutilation is far more widespread and is arguably just as horrible as female genital mutilation.