Everyone here is completely missing the point. It wasn't the API change, the 'new reddit' UI change, or frankly any other individual change. Those are symptoms of a greater problem - Reddit is social media that
succeeded.
This exact same fall happens to any and all social media that succeeds, and is not in any way unique to Reddit.
It grows, and with growth comes complexity and greater expenses to keep it all propped up. In order to pay for those expenses, advertising revenue must increase. To increase advertising revenue, the site must be more 'family friendly' and have stricter moderation. More users means that you can't be as personal and must be more automated. You don't want bad publicity because that can turn advertisers away. If you want more advertising revenue you need more users, which means you need to sand off any rough edges and unique appeal and instead appeal as broadly as possible, regardless of the original intent of the site. To appeal broadly you must add every feature that everyone else has and forget being unique. Broader appeal brings in people who reduce the quality of the content. The larger the site gets, the more appealing it becomes to bots and propoganda. In order to maximize impact for either personal (ego) or professional (money/political) reasons, you need to post content that hits people where they're vulnerable - cute, funny, infuriating, etc.
So, the product experiences enshittification. It's just inevitable. It will always happen to social media if it grows.
You can have a small, niche social media that is good but will never grow - or you can have a large, casually-used social media that is awful. There is no in-between. Anything in-between inevitably slides towards one or the other.