Can someone explain the meaning and function of “as” here:
Mediated by the hallucinatory hellscapes of Chicago drill as pioneered by Chief Keef, the UK version is essentially a South London chapter of trap.
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Chicago drill is a subgenre of rap. It, or some particular version of it, was pioneered (first introduced), by Chief Keef, a rapper.
as pioneered by Chief Keef is a supplement that modifies Chicago drill, letting us know that CK introduced it.
One might say Chicago drill was pioneered by CK; that could be a separate sentence. Here, that information is incorporated into another sentence as a supplement. That is the sense of pioneered; in the OP sentence it is included as a participle instead of as a separate verb.
The OP sentence could have omitted the word as, to say:
Chicago drill, pioneered by CK, ...
Including the word as could mean either that there are other versions of Chicago drill than CK's, or just that this subgenre may not be familiar to the reader and the writer is introducing it as such.
Hmm. It's a common enough expression (though the participles that can be used with it are fairly limited) but I'm struggling to characterise it, and I can't find it in the OED either.
Here are some other examples of the construction, from the iWeb corpus:
... will bring to life Helen Edmundsons show, as directed by Natalie Abrahami.
due in great part to the popularity of the title song, as recorded by Rod McKuen.
A turnover shirt collar with long points, as worn by the actor John Barrymore.
It's similar to a relative clause ("that is/was") but it can be restrictive or non-restrictive, and it has some semantic limitations that I can't quite grasp. It's often about something created or expressed.