Never replied here before. But have something that may help.
I downloaded opera mini in 2015 on an old phone (irrelevant), and if you go to the App Store, your account (profile), and then purchased apps. You can scroll to opera mini if it was downloaded ever before.
As to answer the question of “why is opera mini no longer on the App Store?”
It’s because opera mini can play YouTube in the background without the purchase or YouTube red. Apple gave every web developer the choice to either remove the feature or be booted from the store. Opera chose to instead make the new “opera touch” app, and left “opera mini” in the countries they were aloud to.
If you were to go to App Store > account> country/region.
You could change your region to whatever region still has it listed on their App Store. As to of a list of what countries still have it, I do not know.
Best of luck everyone.