I once did a comparison of battery impact between the Magic Keyboard and the Smart Keyboard Folio with my 2018 12.9 iPP. I used both in succession for a full day. I repeated this for several days. I expected to see more impact with the MKB because of the backlit keys. But the difference was so minimal that I could not definitively state that there was a greater impact with the MKB than the SKF. Overall there really is no issue. The time the screen is active is far more a battery factor than the keyboards. Just don’t have your backlighting set to max.
In fact the only reported and confirmed battery issue was because of a software bug that occasionally caused the backlight to stay on even when the the iPad was in sleep and inactive and the case closed. It’s still a good idea to pay attention to that when putting away the device. Old bugs tend to come back, though I haven’t seen this occur in quite awhile. Also one reviewer mentioned that leaving his Apple Pencil on the charging side of the iPad noticeably drained his battery. But that’s not really related to the keyboard.