But with the current spec, IIUC, it's up to the AS to provide handles for future client reference - so the burden of allowing for smaller requests in the future

But with the current spec, IIUC, it's up to the AS to provide handles for future client reference - so the burden of allowing for smaller requests in the future falls on identity provider software, not the client. And when a side 'MAY', experience says, they almost never do, unless it makes things simpler for them. And having to store extra, global data is something no-one really wants to do. Not to mention handles introduce state, not allow for statelessness: not only the AS now has to keep a global state across all endpoints that may serve a given request, but also the client must keep a local cache of resource -> handle(s). Retry/restart logic has to be implemented, cache clearing logic must be implemented, state has to be kept between restarts of both sides, etc. This is definitely stateful.