You can't completely turn it off. You can reduce the effect in System Settings > Appearance, select Tinted.

If that's not enough, go to System Settings > Accessibility > Display and turn on Reduce transparency, and if that's not enough then turn on Increase Contrast (if you turn that on first, it forces Reduce Transparency to be on as well).

Regarding reverting to Sequoia, I would not recommend that. Reverting to Sequoia is not simple. You need to boot into Recovery OS, erase your internal drive, then install the older macOS, then restore from a backup made prior to the Tahoe upgrade. Even then, the firmware on the Mac may have been updated, meaning potential conflicts and a possible need for DFU mode and a Configurator revive/restore, which would need to be done via another Mac connected to yours. If that sounds complicated and risky, then you're assessing the situation correctly.