I know everyone is assuming (as I was) that BF1 would only have scopes on sniper rifles, and for everything else we'd have to deal with obstructed iron sights. In just doing a little research (something I'm assuming/hoping DICE has already done) I found out that the reflex sight was invented 14 years before the start of WW1.
"In 1900 Grubb invented the reflector or "reflex" sight, a non-magnifying optical sight that uses a collimator to allow the viewer looking through the sight to see an illuminated image of a reticle or other pattern in front of them that stays in alignment with the device the sight is attached to (parallax free).
This type of sight has come to be used on all kinds of weapons from small firearms to fighter aircraft. It is also at the heart of all modern head-up displays. During the First World War the Grubb factory was in demand for the production of telescopic gun-sights and during this time he was credited with perfecting a periscope design for Royal Navy submarines."
(and DICE, if you didn't already know this, it might be a nice thing to put into the game)