In settings, you are looking at extensions page

Thanks, but.... I think is unnormal for clean usability.

Exceptions for the built-in adblocker can be added in settings.

Where?

you explicitly enable access for the extension in opera://extensions

But not for Opera Adblocker.

And why not a word in changelogs?

There are a few things listed in the changelogs at https://blogs.opera.com/desktop/changelog-for-57/. They're kind of subtle.

I don't remember if there was anything mentioned in any of the blog posts, but I did see some things about this mentioned in the comments. Guess there could have been some explicit FYI about it. Hopefully that will improve in the future.

For a long time I could not understand why gestures do not work

Opera's built-in gesture support or some gesture extension? If Opera's built-in gesture support, it should be excluded from the blocking policy. If it's not, that's a bug. If you're talking about an extension that you added to Opera, then yeah, I can see why it (and adblocking extensions) not working on yandex.ru could be confusing if Opera doesn't tell you why in some way.

It's interesting to know why this flag was implemented. chrome://flags/#search-results-extensions-block

It's done for privacy reasons so extensions don't have access to search engine result pages unless you explicitly enable access for the extension in opera://extensions.

So why did you mentioned it?! Or did i misunderstand something?

I do not understand why the introduction of this flag? And why not a word in changelogs? Should I wrestle with why the browser behavior changed?

Mouse gestures not working is totally unrelated.

I do not use embedded gestures.