Long flu is a thing [0] [1]. People were talking about that for years after the 1918 flu.
There's some evidence that a prior flu outbreak, the Russian Flu around 1890, may have been an earlier coronavirus pandemic [2], but nobody could differentiate them at the time.
EDIT: The parent's question seems reasonable. Covid is not unique amongst viruses in the damage it causes. Besides long flu there's things like myocarditis [3] from the flu that may have been neglected in the past, also.