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.
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.
We've always had marginal/debatable illness. The existence of Chronic Lime Disease is debated, similar to Fibromyalgia, or perhaps Chronic Fatigue. Havana Syndrome is a more recent example. It's possible that these are actualy manifestations of psychological conditions like depression/anxiety, but people have a hard time accepting that. Doctors debate this, and I'm not a doctor. Who knows, maybe these are lingering conditions brought on by flus or other viruses.
To say that the Flu has not been under the same level of scrutiny as COVID-19 is ignoring 1500 years of human history.