On my son's 6s, the battery life seems to be the same on iOS 11 and 12, in other words not so great and noticeably worse than on iOS 10.(Battery health is at 89%)
When iOS 11 came out, there were numerous long threads here and on other forums about how iOS 11 ruined battery life for iPhones 6s, 7, and etc. I don't see anything like that with iOS 12. It's possible that the battery life is actually ok on iOS 12, but it's more likely that people upgrading from iOS 11 to 12 don't see much difference - I don't see anyone commenting that the battery life has noticeably improved with iOS 12.
I'm thinking about keeping my 6s on iOS 10.3.3.