Using older macOS on MacBook Pro with updated EFI firmware If, for example, I'm using El Capitan on a MBP, I then install High Sierra on it and the firmware gets updated, will I be able to re-install El Capitan  if I decide to do so later?
Yes.

It's not the firmware that dictates what OS can be installed, but the Model identifier (i.e. MacBookPro 12,1 for a 2015 13" MacBook Pro).

If your Mac was capable of running a macOS version before, it will continue to be able to support it after a firmware update.