said by Fleeced:
20/2 is not "wideband" and does not require a docsis 3 modem.
Wideband is only a word used to describe higher speed tiers because of bonded channels to reach a certain speed. You can have a standard tier with Docsis 3 modem and bonded channels. Bonded channels is not limited to what tier you are in, it's the system that provides the channels and if you have a modem that can use all channels, you are using wideband.
It's for future proofing why TWC is recommending D3 modem specially for subscriber's owned modems. My guess is, TWC will be bumping the turbo speed in the future above what D2 modem is capable of. For renters who wants to stay with D2 can easily exchange their modems when the speed bump happens, it's their choice if they want to wait but I'm pretty sure if you are in turbo renting the D2 modem they can easily exchange to D3 modem.