Those particular channels could have been moved off of SDV, especially if they are highly viewed channels in the area, but in general...yes eventually most all channels will be SDV to free up bandwidth for data.
SDV is definitely still necessary - more and more channels will be moving to SDV since it uses significantly less bandwidth. And if the device needed a TA in the past, it will still need one.
As for the TiVO not seeing the Tuning Adapter? I would say "if it isn't broken, don't fix it"... But if you really want to test the theory: unplug the TA, plug the TiVO straight into the coaxial outlet, and see if it still works.
I've used tuning adapters with my Tivo HD (no longer in service) and my Tivo Roamio Plus for the past several years to get all of my TWC channels. When there was a problem with the SDV, Tivo would provide an error message. However, I started noticing at least a few months ago that my Tivo Roamio seemed to stop using the SDV and I did not provide any error messages.
Finally, a few days ago, I took a closer look at this. The SDV is still connected to my Roamio and there definitely are no error messages. So I did a test by trying to record 6 different channels simultaneously (i.e., 6 channels that previously required the SDV -- such as USA, History, etc.). ALL 6 channels recorded perfectly at the same time without any issues. The Roamio setup screens definitely indicated that there is NO tuning adapter connected even though one is "physically" connected. Therefore, it certainly looks like no tuning adapter is needed. I tried searching the internet for some corroboration but could not find anything that would confirm that an SDV is no longer necessary. Can anyone else confirm this? Is there any confirming info somewhere on Spectrum's website (if there is, I can't find it)? By the way, I'm still using Tivo Experience 3.