Hi avayax,
As stated above the "S" models replacements are for the T4 Series only (currently).
I work for a company who shift a lot of Yealinks (and I mean a lot) and the T23G is one of the most popular phones we sell (due to it being a good solid mid-range phone).
We have not yet seen anything on the road-map to state that this phone is to be replaced anytime soon (I know they have been concentrating their efforts in other areas) but obviously at some point we will see a replacement.
That being said just because the model goes EOL, normally 3CX still provide supported templates for them so you don't need to worry about anything there.