They now adjust max aim level (think I called it ‘sight dispersion’) and aim speed. Basically more open sights let you aim faster but have a limit of how well they aim, while others allow very precise aiming, but are slower.
If you have several, it uses the fastest one that can improve your current aim level, so you can get a rough aim with iron sights, then switch to a scope to aim the rest of the way to get a long-range headshot.
What they no longer do is make the gun any less accurate, or stack with each other.
