The latest MUTCD and states following it to a "T"

[J N Winkler]

Underlining?  AFAIK, California does not underline anything on guide signs.

It is underlining in the special sense used in the Hurlburt paper.

I've noticed that Kentucky is dividing destinations with white lines, and sometimes mileage signs as well. Is this new in the MUTCD?

It is not.  There was a 1962 paper by the UCLA psychology professor Slade Hurlburt, one of the big names in midcentury human factors research, that found it was beneficial for driver comprehension of destination-and-direction signs.  I believe it resulted in California implementing "underlining" (use of ruled horizontal lines to group destinations according to direction) for such signs.  However, the typicals for destination-and-direction signs in Standard Highway Signs have never used grouping of this kind, and I think the MUTCD is silent about it except possibly as a mentioned option in Chapter 2D (the conventional-road guide signing chapter).  California also does not use underlining on mileage signs.