@marcolackovic,
@faraza is correct. You can no longer mention or tag someone on ayoutube-channelif you decide to reply directly in the main comments section.
Alternative Solution:
The following is someone’s comment thread:

As you can see, if you reply to the main comment, the tagging feature is not available.
I have a workaround... You can still tag someone on YouTube, but only if you click REPLY
to them in a comment thread.
Here is how to tag someone in a Youtube reply to a comment...

You can now see what happens when you click reply
to someone in the comment thread. It will automatically tag the user in the comment you are writing.
Now, post a comment and see what happens to the tag.

You will see that after posting the comment with an auto-tag, the username is tagged in the post and appears blue. That’s how to tag someone on YouTube.

Hopefully, this gives everyone some more clarity as well as an inside tip/trick alternative if you want to tag or mention someone in the video comments in spite of YouTube not having a direct way to mention or tag others.
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