I always felt the "Golden Age" of DOS games was 90-95, this archive at least mostly agrees with that feeling, which is interesting to me: https://imgur.com/a/gfO19Nt
The outliers are interesting to me too:
Adventureland (1978)
Beneath Apple Manor (1978)
Pirate Adventure (1978)
Mission Impossible (1979)
Dungeons of Noudar 3D (2018)
Encroaching Gloom, The (2018)
MagiDuck (2018)
Planet X3 (2018)
Sigil (2019)
I'd interpret the question as last (mainstream) DOS game made. As in sold in retailers and therefore somewhat known. And IME it's probably the WWII shooter that used Build engine.
It's a long one, but it tries to answer when the first video game was made. Most of it, though, is trying to figure out what it means to be a video game in the first place. Sometimes really the trouble with the question is the lack of precision in the definitions involved.
The outliers are interesting to me too:
I've heard of a couple of 'em.