I was just looking at my synaptic history searching “microcode”. Here’s almost 2 years worth…
Aug 25 2017 intel-microcode (3.20151106.1) to 3.20170707.1~ubuntu16.04.0
Jan 11 2018 intel-microcode (3.20170707.1~ubuntu16.04.0) to 3.20180108.0~ubuntu16.04.2
Jan 22 2018 intel-microcode (3.20180108.0~ubuntu16.04.2) to 3.20180108.0+really20170707ubuntu16.04.1
This does not directly affect Additional Drivers but shows how the Jan 11 update was backed out Jan 22 (note the creative versions). I’ll bet that’s when it disappeared in Additional Drivers.
That Jan 11 update did cause problems (none here, however). So I’m sure microcode control via Additional Drivers was changed a bit.