For United, this is not a policy. UA has specifically instructed FAs that it IS NOT a requirement that we have pax raise their shades during takeoff & landing. Many FAs have been erroneously telling pax to do this and it must have somehow gotten back to management.
Personally, I still instruct passengers ONLY at the WINDOW EXITS to raise their shades, and I will continue to do so. The reason being is that in the event of an evacuation you must ensure there is no obstruction, smoke, fire, or debris near the exit door. You can only see any of this if the shades are up. If there IS any fire, obsruction, or debris, you obviously cannot evacuate from that particular exit. This is drilled into us during emergency procedures training.
However, regarding non-exit window shades being up, the rationale for FAs insisting upon this is that if pax notice any peculiarities (such as smoke, fire, or anything else unusual on the tarmac), they can readily inform a crew member. We often rely on passengers to be our eyes and ears since we cannot see anything outside during taxi.
And for that reason, many of United's FAs still disobey that policy and continue to instruct all pax to raise the shades. Again, I'll only do it if it pertains to overwing window exits. Sorry, boss.