The degrowth mindset has existed for at least 50-60 years on the left, it merely bubbles to the surface more prominently from time to time. It was extremely popular ideologically in the late 1960s and 1970s.

For some on the left, they view humans as parasites. It starts as self-hate internally and expands outwards to encompass humanity more generally ('I love animals, I hate people'). Humans have to modify their environment to exist (shelter, farming, hunting, et al.), so the people that hold that ideology, you will find, they comprehensively hate any modification of the environment by humans, ie they want humans to cease to exist, rooted first in their own desire to not exist (they hate their own existence first and foremost).

The world is at all times overflowing with people that hate existing. They're miserable sad sacks and they desperately want everyone to drown in it with them. Sometimes that gets formalized into projected ideology used as a weapon against the rest of humanity.