There was a fair amount of UI discussion of this (in 2016) at https://community.metabrainz.org/t/merge-vocals-into-instruments-style-609/105524

A cool thing would be if one could set "can be used on" and then we'd have attributes;
"programmed", "instruments", "vocal" and then it would show in those "trees":
it'd be multiple choice, it'd solve things like programming instruments and "singing" for vocaloid.
So then we could keep the interface as-is but add checks for what relationships they can be used in so vocals still only appear under vocals, no acoustic instruments would show up on "programming", things like "computer" (INST-589) would only show under programming, etc.

Could even be extended for things like arrangement or generic gear, (https://community.metabrainz.org/t/amp-as-instrument/335394/12 ) to really widen our abilities.

to expand on this:

My idea is to have one tree, but each instrument can have any or all of 4 attributes (ticket box): “instrument”, “vocal”, “gear” and “programmable”. Then, in the edit-relationship search for attaching vocals/instruments etc, to the “artist performed [instrument]” relation, the instrument would only show up at the appropriate places:
say “Electric guitar” has “instrument”, it wouldn’t show up in the “vocal” relationship search, but vocoder would have “vocal” and “programmable” and thus would show up in both, “Computer” would only have “programmable” but “soprano” would only have vocal, various synths would have “instrument” and “programmable” while a dampener could probably have “programmable” and “gear”… and so on.

Basically the relationship adding thing would look the same, and the drop-downs would be "vocal", "instrument"(and then "gear" when/if we get that, and "programming" as before (atleast until/if we get a redesign here))
But what is shown in each option is determined with the attribute of instrument
Later we can merge all the performance types and also programming to one thing and then have it like the current box for adding instruments to the instrument performance relationship, but the box will also have "add vocal", "add instrument", "add gear" etc.
But for each search you'd type and in the "vocal" one you'd only see the instruments "tagged" (had the attribute set) as "vocal", in the "instrument one" you'd only see instruments tagged with instrument, etc
This attribute can be set n:m, so then vocoder can have both vocal and programmable so people can be credited as "programmed vocoder" as well as "performed vocoder" (or vocaloid)

typing it here since it seems a better place for it

I thought about this a bit more and I think we should still keep them separate like now in the UI (at least initially, and for as long as people are happy with it).

Additionally, I'd also split ensembles into a third attribute (which would also include orchestras). This one would be shown for "performed X", but wouldn't be shown for "member of" - an artist shouldn't be member (string quartet) of another.