I’ve been missing this for a long time as well. Readmill was my favorite ebook platform because it had highlighting that could be shared and followed you across platforms. But I’ve not seen anything work yet across ecosystems: i.e. only this ebook platform or only on the Medium site. There has been some work done at the W3C on an open annotation standard, which was in part instigated by what Hypothes.is has done. I worked on a site for Science in the Classroom that implemented it for ‘playback only’ that uses it in an interesting way, but the annotation data model that the W3C laid out would be a very good start to implementing it at the OS level, which is by far the most useful.