Trying to resolve whether thread drift is appropriate is a bit like shaving Schrödinger’s cat with Occam’s Razor.
In many ways, thread drift is an integral part of the haecceity of FTC. it defines so much of the content we create. A thread that starts off about pizza at Vito’s may drift into discussions about the various regional styles (NY, St. Louis, Chicago, etc), or about whether canned mushrooms are ever an appropriate topping.
Or create a thread about Dan Dan Mian and it’ll inevitably spawn discussions about whether authentic Dan Dan Mian is dry or wet, or which restaurant makes it the best, or whether this is really a Sichuan dish.
Much of the unique appeal of thread drift is that it’s a way to naturally explore a particular food, or dish, or whatever. In a way, it’s as much exploration of the food itself as it is self-exploration. In learning about the culture or history, or some other collateral aspect of a particular dish or food item, we learn as much about what makes a particular food item special (i.e., what makes pastrami an integral part of Jewish deli culture), as we do our own preferences, biases, and just plain ignorance. And one cannot really begin to comprehend how much they do not know, until they start to understand what they do know.
You will never get those thread drifts if you intentionally tried, they have to happen organically. And if you separate the thread drift from the original thread (even if a moderator did it so that there was a link in the original thread to new “thread drift” post), it will most likely wither as what made the thread drift interesting and worthwhile in the first place was the context in which it was spawned.
Like finding a yellow rose in red rose bush. Snip the yellow rose from the bush, and it’ll die even if you plant it in the ground.
Of course we can simply ban thread drift entirely, which is fine. It’ll make the site different, but no less worthwhile.
My only concern with banning thread drift is that it puts a chill on forum participation. Way back when we all hit the escape hatch from Chowhound and found refuge elsewhere and then eventually here at FTC, a large part of the motivation was to find a place where participation could be uninhibited, and without fear of recourse from up above. A place kind of like Cheers - where everybody knows your name and you can just hang at the bar as long or as short as you want and shoot the shit with a bunch of folks who share your passion (food).
I just think at the end of the day most, if not all, of us have enough self-awareness to know when thread drift is kosher and when it has just become a complete fucking waste of time.
Although I think our Vegetable thread did drift off into other topics, but it was all good. 
Hope you enjoy the stay.