I did some more research on the issue, and found that the exemptions for sleeping in the sukkah are not so modern.
Of course. This has been mentioned several times.
However, to say that it became a minhag not to do so is just ignorance.
It became a minhag in *Chabad* — that’s my point.
Just as eating outside the sukkah on Shemini Atzeres is not permitted according to the Shulchan Aruch, but became a minhag in other Chassidic communities (not Chabad).
Just as lighting the menorah indoors was only permitted by the Shulchan Aruch in times of danger, yet became a minhag among many Jews (as cited in the kovetz brought by Daas Yochid).
All three of these practices have complex pilpulim to be melamed zchus on the minhagim—pilpulim that wouldn’t stand on their own, but serve as support once these customs were practiced by Gedolei Yisroel in their respective communities.
The reason I brought up the Shulchan Aruch is simply to show that, of the three minhagim, sukkah has the most leniencies in Shulchan Aruch — yet it receives the most criticism in these online forums (I wonder why…).
Even according to the opinions that there are exemptions, if those don’t apply, then one should sleep in the sukkah.
While you might imply that from the Shulchan Aruch, it doesn’t state this explicitly. It doesn’t say that one may only sleep outside the sukkah if they are married or it’s cold—rather, those are offered as *reasons* as to how the leniency came to be.
In fact, there are poskim who clearly write that these leniencies can be extended to others. For example, the Eshel Avraham writes that bochurim may sleep outside the sukkah, since they can rely on the fact that “רובא דעלמא” most people are exempt due to their wives.
Contrast this with the menorah: the Shulchan Aruch explicitly states that lighting indoors is only permitted in times of danger.
P.S. I’m not citing the Eshel Avraham as the source for the Chabad minhag. Regarding Chabad, everything I’ve said until now stands on its own. I’m simply contrasting sukkah to menorah: the Shulchan Aruch does not clearly state that the leniency of sleeping outside the sukkah applies only to married people or in the cold — whereas with menorah, it does explicitly say that the leniency applies *only* in times of danger.
In other words:
Shulchan Aruch regarding sukkah: “And regarding the current practice to be lenient with sleeping – that people do not sleep in the sukkah, except for those who are particularly meticulous about mitzvos – some say it’s because… But it appears to me that it is because…”
Shulchan Aruch regarding menorah: “The Ner Chanukah should be placed at the entrance closest to the public domain, on the outside… And in a time of danger, when one is not permitted to fulfill the mitzvah publicly, he should place it on his table, and that is sufficient.”