IETF Consensus is the IETF's way of agreeing to some action. The IETF
makes that decision, the IESG (currently anyway) is tasked with
determining whether or not the IETF has actually made a decision. IETF
Consensus clearly requires a last call be made. It does not (of itself)
require publication of anything at all - publication may be made to
record the consensus, it does not cause it, and is not required to obtain
it.
Given that a large portion of the IETF does not in fact subscribe to the
ietf-announce list, and that in some cases the IETF consensus is pretty
obvious (for instance when the decision is just paperwork following up on
another IETF consensus decision), I wouldn't even say that a Last Call is
always required.