I don’t think this is something that should be handled by the UNA core. Opening links in a new window would have to be done with JavaScript, because the timeline uses AJAX and the links are generated dynamically, if I’m not mistaken.

I also can’t really imagine every timeline item opening in a new tab. With a larger number of users, that would mean dozens of open tabs, which would probably make the site harder to use.

So yes, technically it can be done with custom JS, but as a global UNA feature it probably wouldn’t make much sense. Maybe I misunderstood what you meant.