Have you seen this joke in this scene? If so, where? Some particular TV network or streaming service?
This isn't really a "deleted scene". Maybe more like an "alternate scene". I came across a random old Reddit post about being surprised seeing this scene for the first time while watching 30 Rock at a friend's house or something. They were streaming the show on ITVX, which apparently uses this joke in the scene instead of the "this is how white people dial a phone" joke that is in this episode seemingly everywhere else. As if they had filmed a couple different jokes that both provide the same underlying exposition (i.e. implying to the audience that Tracy actually is part white), and had planned to decide during editing which came out more funny.
So, I signed up for a trial and yes, this foot joke is here. There is no further explanation or anything as to exactly why Tracy has a white foot. It just cuts from this to the reactions of Dr. Spaceman and Frank, then proceeds to the next scene between Jack and Devon. I think the reason for the change on ITVX is simply to decrease the run time of the episode. My Blu-ray copy of the episode is about 26 minutes long, but on ITVX, it's about 21 minutes. The phone-dialing joke is definitely several seconds longer than the white-foot joke, so my guess is they switched to the less funny alternate joke for its shorter runtime.
Seeing that runtime made me think of Peacock, which is notorious for cutting 30 Rock episodes down to a 21-minute runtime. Each episode tends to have many little things cut out, just like another thing that ITVX did with this scene: cutting Frank's "Yeah, but that guy was into black chicks. I'm surprised I'm not a descendant" line down to just "Yeah, but that guy was into black chicks." So I checked Peacock, and, unlike how almost all episodes are 21 minutes, this episode is actually 26 minutes there (just like the Blu-ray). I am not sure why they choose some 30 Rock episodes to play in full on Peacock (like "Jack-Tor" that season) while all the rest are cut down.