Enshittification refers to offering the same service (often free, or at least with an option to pay more) but making it worse in order to squeeze you onto a paid (or higher paid) tier of service
It doesn’t have to be a paid service, it can also refer to (and usually does) a two-sided market. For example, a site with free users and advertisers. The platform first gains a critical mass of users, then they switch to focus more on the paying advertisers to increase value for shareholders. Over time, the main focus becomes the advertisers.