Because they’re so similar, I’ve decided to discontinue the custom highlight posts my site had in lieu of the more prevalent annotation post kind. The layout and format of both as highlighted text quoted from another site was almost exactly the same with the primary difference being my additional commentary added to the highlighted text to call it an annotation. Conceptually I considered “highlight + commentary/reply = annotation”. The difference is marginal at best–pun intended.
Since I only had 13 highlight posts versus 121 annotation posts (plus various additional annotations and highlights which I’ve rolled up into the body of some of my read posts) over the last year and a half, I felt it seemed redundant and bothersome to maintain two separate, but nearly identical post kinds. Semantically one may think of a highlight on some text as an annotation anyway, thus the idea of annotation subsumes that of a simple highlight.
As of this evening, I’ve changed all the custom highlight posts to be of the annotation kind. Other than the one word visual difference of the post kind text changing from “highlight” to “annotation” this change won’t affect much except for those who may have been subscribed to the highlight feed. Going forward you may consider subscribing to my annotation feed instead.

I had created highlight posts first, but in the end annotation posts have won the day. And for those that don’t have them, fear not, because honestly annotation posts are really just glorified bookmarks with custom text in the context. (The glorification only entails a highligher icon instead of a bookmark icon and a bit of CSS to color the text yellow.) I do find having them delineated for my personal research purposes useful though.