Is Winter’s Envoy coming back?

Guess so (and hope so).

Reruns aren't put on the calendar.

And if you looked at the Tempest Notifications, you should see the 1st winter units in the Bonus Unit image. That should tell you something.

They're shown as this TT's bonus units in the notifications so that's a hard confirm (unless IS wants a mob to come for them). I honestly thought it would've been here today but my guess is they will come tonight or together with the new banner.

They reran all the precious year's banners, so odds are certain that the banner will return.

It will come back on December 18th, the same day TT begins.