[troupo]

> There's no point in idempotency for operations that change the state.

Of course there is

> DELETE is supposed to be idempotent, but it can only be if you limit yourself to deletion by unique, non-repeating id

Which is most operations

> Should you do something like delete by email or product, you have to use another operation,

Erm.... No, you don't?

> which then obviously will be POST anyway. And there's no way to "cache" a delete operation.

Why would you want to cache a delete operation?