From this question I learned that "many" people don't make Kiddush between 6-7pm.
Who does this? Is this primarily a Lubavitch Minhag? If so, among Lubavitchers, how prevalent is it?
Among Lubavitchers, at least, this is very common. It is certainly the case in every yeshiva that I have studied or worked in. All the homes I've eaten in on Friday night observe this.
That being said, its reasons and particulars are not well known at all. There is some confusion in the general populace about whether it applies only to red wines (something the Alter Rebbe does not mention at all), why it is not calculated with shaos zmanios, what's its reason, etc., etc.