Patrick Steinhardt [off-list ref] writes:

I wonder whether we can future-proof the code a bit more by introducing
a struct that contains everything we want to pass to the callback
function.

That would hopefully make a change with a large blast a one-time
event.  But at the same time, it may end up making it too opaque and
hard to verify if all the API functions are using/updating/verifying
all the members of the struct as they should.  Compared to that,
unused parameters are easier to verify mechanically by compilers.

This would also allow us to get rid of this awful `peel_iterated_oid()`
function that relies on global state in many places, as we can put the
peeled OID into that structure, as well.

Yes, such a benefit may justify a one-time "affect many callers"
event.  Or the underlying for_each_*() friend of functions can be
updated to use a single struct and then the current "only selected
parameters that are used ar passed" API can be made into a set of
thin wrappers around it, and then callers can be converted one step
at a time, in a multi-step series, perhaps.