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AFAIK, only support gems can change what other support gems can apply. Uniques can't do that. Hence the duration effect on old Burning Arrow because Pitch Darkness exists.
Though I might very well be wrong. /u/mark_ggg, can you confirm?
This is correct. Only support effects can change skills in ways that affect which other support effects can apply. Because this isn't a support effect, it can't change which supports apply to the skill.
Okay but hear me out... what if it were a support effect. plz?
We don't currently have the ability to do global support effects. We got some of what we needed when we implemented global gem level modifiers, but support effects are more complciated than those. So we can't currently do that. And it'd be a lot less interesting only affecting socketed things as an unset ring.
What about Pitch Darkness? That allows Burning Arrow to benefit from Less Duration.
No, it doesn't. Burning Arrow always benefits from Pitch Darkness, precisely because we can't make the stat dynamically enable this behaviour, so it had to always be supportable by duration supports.
This is less weird in 3.9.0, since it has it's own debuff with a duration anyway.
Wait so all this time we thought the jewel was enabling it...
It was definitely known in the past by players that it always could benefit from the support - there was a whole lot of annoyed players who complained that was the case a while back when a streamer demonstrated it.