Don't remember, maybe on Reddit. But I have been searching for a long time too. And I there is no way to get rid of other search engines. But obviously, this is not bothering me at all. I just don't use them.
In the AdBlock section of Opera settings, disable Automatic cookie Dialog option.
It's in the AdBlock section that has the option to bypass those website cookie notifications. For some sites it doesn't work yet. So you must accept cookies by hand from then on.
@kenriley001 With beta 54, the behavior is slightly different. Regardless of adblock setting, the "can't be reached" message frequently appears on the first attempt at download. Closing the tab or closing Opera, then retrying the download is successful.
It appears there is something peculiar about apkmirror. I have never been able to whitelist this site with Opera, and a Google search reveals this is a long-standing problem with some ad blockers.