Yes, you can wear leggings on United (but airline can police your clothing).

Internet Erupts After United Airlines Boots Girls for Wearing Leggings

Do female police officers wear pantyhose/stockings while on duty? - Quora

United Bans Girls for Wearing Leggings

Why United Airlines can get away with treating its customers like garbage - Vox

Can guys wear leggings like girls? - Quora

Do you wear tights or leggings under your dress? - Quora

Is it fair that airlines impose a dress code on free flights?

Yes, Leggings Are Pants and If You Don't Agree, You're Wrong

Can we wear knee caps under leggings? - Quora

United Airlines defends gate decision to bar girls wearing leggings from flight, Air transport

Can you wear leggings on a plane? - Quora

Can You Wear Leggings On A Plane? – solowomen

United Airlines says customers are 'welcome' to wear leggings — unlike teens who used special passes - Los Angeles Times

Here's the Reason a Group of Girls Wearing Leggings Weren't Allowed to Board a United Airlines Flight

After Barring Girls for Leggings, United Airlines Defends Decision - The New York Times