I had to get a replacement 6s Plus in May and restored from back up and noticed a definite decrease in battery life. After DFU restoring it and then setting it up as new I had no issues whatsoever getting the same (actually better) battery life as the 6s Plus it replaced.

Does restoring iPhone help battery life?

First off: By resetting only the “Settings” on your iPhone, you can retain all data, apps, media and chat messages, contacts or call logs. At worst, your battery life is still be acting up after this attempt, but you won't lose any data.

Does a factory reset improve battery life?

If wiping the cache partition still doesn't fix battery problems after a software update, your very last option is to do a full factory reset. This will erase every app installed on your phone and their data/files, leaving behind a phone that feels like new.