If that fork of the client does indeed exist, it is before my time at Hypothesis, so perhaps someone else might be able to point you toward it. In the meantime, I can answer your question about why we haven't added an upvoting/downvoting feature to the product yet.
Recently, we've seen an incredible amount of growth in the education space, so that is where we have been focusing our development efforts: LMS integrations with grading, accessibility and security work necessary for our education users to get approval to use the tool, etc.
Upvoting/downvoting isn't among the most requested features in general, and in the education space specifically the idea has even met some resistance (some of our teachers like that there is none of the social pressure that can be associated with upvotes/downvotes).
That doesn't mean we will never implement upvoting/downvoting (or a similar feature). It might be something that is configurable (so teachers and other group creators could turn the feature on or off), or there might be another way to bring this feature to the client that lines up with education and non-education use cases alike. It likely won't be a priority for us in the near term, though.