I personally find it useful for the reasons I gave above. Given that other members of the community sometimes complain that submissions aren't new or current, I think it's likely useful to them as well. Submission titles are also changed for other reasons in keeping with the guidelines, such as please use the original title, unless it is misleading or link bait.

https://news.ycombinator.com/newsguidelines.html

Similarly, the "[pdf]" and "[video]" tags are included in the guidelines:

If you submit a link to a video or pdf, please warn us by appending [video] or [pdf] to the title.

It sounds like you don't find it useful, which is fair enough. Do you find it harmful or misleading in some way? If so, how so?

Edit to add: Parent updated with additional rationale:

Mods adding the (publication year) is exactly like saying "hey, I've kind of read this, it's kind of old and you may (not) be interested in it because it's <years old>. I have no idea of the real value of the text but the year may give you a clue".

I think you're reading too much into it. For me, it just puts the post into chronological context. It doesn't give me the impression that it's any less worthwhile to read. In some cases, the year provides additional interest, particularly if it's older.