I’m on 16.04 installed May 2016 and right now, Intel microcode no longer shows in Additional Drivers.
I highly suspect this is the result of recent meltdown/spectre updates that came, went and were modified. Of course, Intel microcode is a key part of mitigations and it’s been a mess, so to speak. Intel put out some bad code and no one trusts them at this point.
EDIT: And this may be processor dependent. Mine reports Intel® Core™ i7-4790 and I believe it’s Haswell.