Games

That's one way it happens, but in general the term appears to be about decline in quality for the purposes of profit-seeking, regardless of whether services were offered for free or not.

The wiki article starts with this:

Enshittification, also known as crapification and platform decay, is the term used to describe the pattern in which online products and services decline in quality over time. Initially, vendors create high-quality offerings to attract users, then they degrade those offerings to better serve business customers, and finally degrade their services to users and business customers to maximize profits for shareholders.

Other articles I looked at seem to agree with this basic concept.

And like I said, spam from scammers and inbox spam are examples of shittiness that seep in regardless of if you engage or not. There is no "no marketplace plz" option, and even if there were scammers can still send you friend request spam.