Incursion and Delve have a variety of drop-only item modifiers. Some are ubiquitous - in Blight league, for example, the "+1 to maximum number of Spectres" prefix, which occurs only on body armours from "Contains minion items" Delve nodes, was highly used. Others are forgotten, in no small part because of their omission from popular third-party tools - I think that for this reason, as well as for convenience, they deserve to be added to PoB. Since it is only possible to have a single drop-only modifier, it might be best to have to declare the item Delve, or Incursion/Temple, or just drop-only, in the same way that declaring an item shaper/elder works currently.
http://poedb.tw/mod.php?cn=Incursion has a list of the various drop-only Incursion modifiers. It doesn't specify base type restrictions, unfortunately, though the list is small enough that memory suffices.
https://www.reddit.com/r/pathofexile/comments/9owjzb/discussion_lets_talk_about_droponly_and/ has something of a list of the Delve drop-only modifers, sorted conveniently by base type.
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