@ctriant please resolve the conflicts.

I know callable already appears in a couple of places. I still think we have too many variants.

this feature it's something we've discussed with @c00kiemon5ter and we decided to follow the same methodology as with token exchange and token revocation.

I don't mind if we refactor or even discard it. What's your opinion on that @c00kiemon5ter ?

What we need is a way to run a custom hook when the userinfo endpoint is invoked.

The immediate goal is to enrich the userinfo response with more data fetched by another remote URL, when a certain scope has been given to the access token.

The idea is to have this as an additional policy to the userinfo behaviour - the intention is not to modify the behaviour of the userinfo handler.

I have nothing against custom hooks. I just think that having basically two variants of hooks should be enough.
We now have class and function (beside callable). Using class allows you to specify additional key word arguments, used when initiating the class, through configuration. It also means that class most support the method call .
function on the other hand is just a function that either takes known arguments or no arguments.
So what makes callable different to the extent that we can not use function or class in its place ?