In https://www.dndbeyond.com/magic-items/unbreakable-arrow the line "Proficiency with a longbow allows you to add your proficiency bonus to the attack roll for any attack you make with it. " doesn't appear in the books. I suspect notes Heavy, Range, Two-Handed are wrong as well.
My guess: This may have to do with the fact that DDB doesn't have a proper ammo item type. It looks like it is branched off of longbow some how for some reason.
In https://www.dndbeyond.com/magic-items/unbreakable-arrow the line "Proficiency with a longbow allows you to add your proficiency bonus to the attack roll for any attack you make with it. " doesn't appear in the books. I suspect notes Heavy, Range, Two-Handed are wrong as well.
My guess: This may have to do with the fact that DDB doesn't have a proper ammo item type. It looks like it is branched off of longbow some how for some reason.
If you look at the arrow of slaying it doesn't have the longbow properties.
https://www.dndbeyond.com/magic-items/arrow-of-slaying
So I would say the unbreakable arrow has errors on the description page.