The UnifiedHighlighter is derived from the PostingsHighlighter, which should be quite obvious to anyone who cares to look at them. There is no feature in the PH that is not also present in the UH. The PH is marked as lucene.experimental so we may remove it in 7.0. The upgrade path is pretty easy given the API similarity. By removing the PH, the goal is to ease maintenance. Some issues lately have been applicable to both of these highlighters which is annoying to apply twice. In one case I forgot to. And of course there is user confusion by having both.
What I propose to do in this issue is move CustomSeparatorBreakIterator and WholeBreakIterator out of the postingshighlight package into the uhighlight package (or perhaps add a common or util should future highlighters need them?). Then of course remove postingshighlight package.