I wonder if all these negative impacts of COVID-19 could also have been associated to seasonal flus in the past, had they been under the same level of scrutiny.
COVID-19 is not the Flu. Also, the Flu has been around for at least 1500 years, so humanity would have observed "Flu Brain" or "Long Flu" a long time ago. To say that the Flu has not been under the same level of scrutiny as COVID-19 is ignoring 1500 years of human history.
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.
[0] https://www.sciencefocus.com/news/long-flu/
[1] https://time.com/5915616/long-flu-1918-pandemic/