Does this have something to do with the OCR software?
No, thank goodness.
What is Tesseract?
Tesseract was a free and open source frontend to Lemmy. Basically, the logic of your instance is handled by Lemmy, but you can replace the look of it, or even give your users multiple options, by adding a frontend.
Key here is that it is reasonable to expect a frontend to present the information from the backend correctly and completely. If you want to filter out communities or people, you would do this by joining the right instance, changing the settings in your client, blocking people you don't like, and subscribing to communities you do like.
And if he really wanted to have a filter in his frontend, it should have been opt-in, not opt-out.
So once db0 (admin of Lemmy.dbzer0.com, who the dev previously clashed with [1]) found and exposed the existence of this filter, which is practically a secret blacklist of every user the dev found annoying.
We've been clowning on the dev ever since.
As an addendum, the dev crashed out and seemingly closed his instance (dubvee DOT org), leaving a goodbye rant. All well and good, but he embedded a bit of JavaScript that basically queries lemmy.dbzer0.com 1500 times whenever you open dubvee! Hence why I'm not putting the dubvee URL, and please don't go there at least for your own safety.
What is the list?
The list is an obfuscated blacklist. Now if he just blocked lots of people from dubvee, we wouldn't bat an eye. However, he added a practically secret blacklist of users and instances (full transparency: both my instance and my username were both blocked, I'm pretty sure my instance is the "instance-that-will-not-be-named" in his rant). Like it's open source so you literally can find it, but you had to query his server to get it I think.
The developer claimed that this was to "save" the Fediverse from "toxicity". And yeah, obviously, there were some pretty toxic users on that list. But honestly...it was really just people who were active and ever posted anything to the left or right of Reagan, although it was dominated by leftists. In particular, a few trans instances were banned.
Why do people want to be on it?
Because it's really really funny 😆
[1] So my home instance (Anarchist Nexus) is part of the Anarchist Flotilla with dbzer0, basically we roll together and we share admins. One of the admins on Anarchist Nexus put "Death to Zionists" in their display name. Unfortunately for the turbolibs at Lemmy.world, this was too unfathomably based for them, so they threatened to defederate from Anarchist Nexus. Plus, a lot of politically outspoken users come from the Nexus and dbzer0, i.e. we were some of the exact people the Tesseract dev wanted to "save" the Fediverse from.