Let me put it this way, if you were to bet on TBC not happening, you'd probably lose the bet.
They sent out polls on how to roll out TBC servers a few months ago and Ion said that TBC is the "logical next step" in an interview.
Not to mention the boost in subs that Classic brought.
The announcement was most likely planned this years Blizzcon, but now has to wait until their digital event in January.
There is quite frankly no proper event to announce TBC servers, Blizzcon 19 would've been too early and on the Gamescon 2020 stream they announced SL release and they are unlikely to have two major announcement on gamescon.
Also, former Nostalrius dev tweeted that TBC Beta is planned for March / April 2021.
Obviously, information can be
1. False
2. Subject to change
https://twitter.com/NanoNost/status/1303392668765085697