So there's this 6-7 year old post:
https://e621.net/post/show/107167/anthro-bed-big_breasts-breasts-clothed-clothing-ec

The tags for artists are: unknown_artist
Even though in picture, there's literally artist's name in top right corner.

Should this be fixed? I'm unsure about rules on very old posts...

There aren't any special rules for old posts. Go ahead, tag the artist name and remove unknown_artist.

Yes, of course. No matter how old post, if there's tags that should be corrected or added, it should be done.

E: I added source there as well.