I'm referring to private add-ons that are private so your ego would be boosted. Not beta add-ons.
There's only few kinds of add-ons I've ever seen kept private.
1. Because Badspot failbins it.
2. Because the gamemode requires a single higher populated server to be of any entertainment.
3. They aren't done yet.
4. It's not user friendly. It requires extensive effort to become functional in a server beyond installation.
5. It's abusable in common server conditions.
6. It stopped working and due to an update, is irreparable.