It's been four times actually.
Bungie has been accused of lifting artwork four times now. Last year, an artist accused the studio of borrowing its designs for a Destiny 2 Nerf gun, which was amicably resolved.
In 2023, an artist pointed out the similarities between their artwork and a piece that appeared in a Destiny 2 cutscene. Bungie resolved this by compensating and crediting the artist.
In 2021, fan art was used in a trailer, which Bungie again resolved after reaching out to the artist in question.
Eternal had Denuvo, but it was removed a year ago, so it probably doesn't have to do anything with the lower numbers on Steam.
My guess is mainly price and RT.