I suspect that Reddit's own automated systems catch a lot of that. There are strong indicators of that type of spam.
I moderate a few small, and mostly inactive, subs. They don't see a whole lot of activity.
On the odd moments I do drop into mod mode (a few times a year, if that), what I see is a fair bit of flagged spam ... and a lot of off-topic posts. One sub in particular seems to have had its topic (a software application of use to Google+ users saving data, which was last relevant on 1 April 2019) with an exam of some sort in India.
Topic drift, relevance drift, and lightweight content displacing substantive posts are far more likely without dedicated, mission-aware mods. AskScience and AskHistorians would be most especially subject to that, as well known examples. I can think of numerous others.
The other issue would be various forms of abuse and brigading, below the thresholds which Reddit's automated systems would detect. Unmoderated forums would far more likely become unpleasant places to participate.
