Any platform with millions of users and dedicated communities will be hard to generalise, but there were countless examples of it happening.

Length of posts has plummeted, "meme" content and "twitter" like language was repudiated while now its basically the main mode of communication.

There used to be "famous" usernames, not everyone agreed with them but most people considered their input valuable, ending perhaps with the famous Unidan incident.

I would admit that having been in the site for 15 years the degradation has been continuos and small communities were much better than default subs from the get go. But the Eternal September post App release has been irreversible and made the site culture absolute trash

Agreed. Moderators would even try to use custom CSS to remove the downvote buttons on their subs to prevent people from downvoting comments they disagreed with, as that was against "reddiquette." But oftentimes you'd want to indulge your monkey brain and punish people who have differing opinions by making the number go down and their comments go gray.