No, they shouldn’t. When subs reach a certain member threshold modding becomes a job for many. Mods should absolutely form a union, but asking for back pay is a stretch. What they should do is asked to be paid moving forward stating that profiting off the backs of volunteers is no longer acceptable not only because Reddit has made modding much harder by giving third party apps the finger, but also because the mods should in theory value their spare time. Another thing is that people posting free content to Reddit without reimbursement should be viewed as an atrocity, even YouTube reimburses content creators once the content gets a certain amount of views.