The ability to flag a channel as digital, analog, or all is new for the XT. The "all" setting for a channel in the XT is equivalent to the only option you had with the T.
There is a great Wiki page that highlights in bold the new features of the XT over the T.
Making a talkgroup (or conventional channel) digital voice only (or, conversely, analog only) was first an option on the 996T. As already noted, it can be done on the XT series, and is also on the Home Patrol 1.