A "couple of years ago" it would have been Economy Comfort, not Comfort+ (which was introduced in 2015). Comfort+ seats includes free alcohol all flights (including domestic), Economy Comfort only included it for free on international flights. So, yes, you would have been entitled to free booze in EC because it was international, but the rest of coach (which is the subject of this thread) did not get free booze. DL introduced free alcohol (beer, wine, and liquor) in coach on all long-haul international in 2013. This did not include flights to Canada, Mexico, Caribbean, Central America, etc. which are all considered short-haul international.
The only thing that has changed is that Comfort+ (formerly Economy Comfort) now gets free booze for both domestic and international flights. The rest of coach does not currently, and did not previously, get it for free on short-haul international.
"which is the subject of this thread"
It is? Where did the OP state that? The OP did not state a class of travel, however we all know that Business/First gives free alcohol. And thanks for the name correction. Regardless of branding, the benefit is still the same to Mexico as I understand it, as the example I provided?