I might be one of the few that was already on their way out. I had been getting sick of Reddit It wasn't the same thing it was when I first joined in ~2011ish. Back then, content was more scrutinized and users were kinder. As Reddit became mainstream, the content slowly changed to reflect that. It started to be more like an anonymous Facebook. I remember it sticking out especially after the Game Stop incident on WallStreetBets.
A few months before the API fiasco, I was banned from a sub because they misunderstood a comment I made as violating their rules. Because I had been banned from another sub recently (I think I had joined a China one then commented in an anarchist one for the lulz), I was suspended from Reddit entirely for a week. I didn't realize that I was doing it, but I used several usernames depending on what content I wanted to focus on. I commented using another username and was permabanned from Reddit entirely for trying to bypass the temp suspension. The specifics might be slightly different since I'm going from memory.
From then on, I would lurk in my favorite subs sporadicall using Reddit is Fun. Once the API fiasco kicked off a few months later, there was a push for Reddit alternatives, which gave me the opportunity to find and join Lemmy. I've been here ever since.
I'm very much in the same boat, also joined around 2011. I didn't leave because of the API changes, I left because the website was degrading substantially as a byproduct of its userbase.
Lemmy contains so much of what made reddit special in the early days. It was primarily tech-proficient people who cultivated a strong community, held each other accountable, and valued science and evidence.
As more users came to reddit, the initial community diluted. Certain subreddits were still special and worth checking out, but the greater whole was too massive for its own good. Plus, I suspect a huge number of new users were teenagers and children, and their comments and maturity reflected that.
I knew it was basically over once I saw comments on subreddits that regularly made the front page with extremely obvious bigotry and racism. Incescent bashing of women. Comments that reflected the vile nature of the shit comments you'd see on Instagram. This was becoming all too common and was not being moderated. The remaining comments felt like washed out circle jerking or a complete lack of critical thinking.
The IPO was the nail in the coffin. No good could possibly come from that for the users of the site. Haven't been there for over a year and have zero regrets.