The Absence of Evidence is Sufficient
Some proponents of blood flagellation argue that the Imams (a) may have performed the rituals in private, and that may be the reason why we have no evidence that they (a) performed these rituals. Therefore we should not deny the fact that these rituals are Islamically recommended.
Any sincere person with a working intellect would not fail to see the flaw in that argument. If the argument were valid then it could be applied to a whole range of actions that have no Islamic backing. Any given Muslim could innovate a ritual and claim that we should not deny that it is Islamically recommended because it may have been performed by the Ahlulbayt (a) in private. For example we could innovate a new prayer with sixty four rakats and justify it by saying: “Don’t say it is not mustahab, maybe the Imams (a) did it in private!”
The lack of evidence in this case is sufficient to justify the proposition that the Imams (a) did not perform those rituals.