Why Orbot isn’t available on main F-Droid repository?
It hasn’t been for years.
It is in the Guardian Project repo, but that was recently removed from the default list.
Even when it was in the GP repo, it wasn’t always updated.
You can download the latest version using FFUpdater app or from here: Releases · guardianproject/orbot · GitHub
Is there any reason why it was removed?
The steps to compile it automatically broke and it wasn’t updated.