There's a long holiday right now but I've poked @Gnivom about this thread for when we're back in office.
There hasn't been a reserved relations slot since sometime in 2014. What you're seeing in recent patches is not related to that.
They removed extra relation slot reserved for the player, not the extra diplomat. And it was done in 1.21, extra diplomat IIRC is still in. (Could be wrong though, haven't checked it lately.)
There hasn't been a reserved relations slot since sometime in 2014. What you're seeing in recent patches is not related to that.