Well, it's gone from unthinkable to inevitable to announced:
https://hub.united.com/united-corona...ght-attendants
We will also make face coverings mandatory for all travelers and will be providing them for free to our customers starting May 4.
Among US carriers, B6 started this, F9 followed, and UA, AA, and DL all appear to have followed suit today. No end date is listed.
Presented without further comment in an effort to avoid going OMNI.