I created an adblick.list file and added just that plain URL to a single line, added it as custom tracker list (tested as well as custom ads list), but did not work. Used ||mc.yandex.ru/metrika/tag.js, ||mc.yandex.ru^, ||mc.yandex.ru/metrika/tag.js$script and a bunch of other ABP/EasyList syntax, but did not work either. Added [Adblock Plus 2.0] as first line and some common dummy comments, but did not change anything.

I then thought that Opera might internally whitelist it for some reason, overriding even custom rules, but any other attempt to block some script does not have any effect either. I also created some test HTML <script src="https://mc.yandex.ru/metrika/tag.js"/> to test blocking this and some other scripts, but does not work.

Of course ad blocking and tracker blocking is enabled, including a bunch of pre-configured lists, which do all work: the browser icon about it is there, I see the number of ads/trackers blocked, I see them missing in sources and the related console entries etc. It all works well with the pre-configured lists, but any attempt to block anything with a custom list does not work for me.

And I missing something, some syntax detail, file format, file extension/name, any such?`

Ah, of course I removed/re-added/updated/edited etc the list in the GUI, relaunched the browser, cleared cache etc on every test iteration. Most importantly I monitored browser developer tools network tab with cache disabled to see the script being loaded with 200, verifying that the exact request URL matches my rule.