The solution for indexed optics used by lens does not work with profunctor optics. Indexed optics are very useful however, even if their index can't be implicitly forgotten like in lens. Here, I experimented with the following compromise:

For each profunctor p there is a profunctor Indexed p i with which we can define indexed optics by

type IndexedOptic p i s t a b = Indexed p i a b -> p s t

This does not compose to the right with Optics or other IndexedOptics, but composes to the left with Optics. If one has an IndexedOptic and wishes to compose it to the right with another Optic or IndexedOptic, one can use

unIndex
  :: forall p i s t a b. (Profunctor p)
  => IndexedOptic p i s t a b -> Optic p s t a b

to explicity convert an IndexedOptic to an Optic by forgetting indices. Despite of a bit of syntactic inconvenience, I reckon that this is a small price to pay. What do you think?

On that note, what would be a sensible place to put an IndexedTraversable type class like the following in the purescript ecosystem?

class (Traversable t) <= IndexedTraversable i t where
  itraverse :: forall f a b. (Applicative f) => (i -> a -> f b) -> t a -> f (t b)