Perhaps a meta question for sharing annotations -- does anyone know if this is an off-the-shelf solution for hosting paper annotations? It reminds me of Fermet's Margins solution but didn't find a clear signal in the source at first glance

I have never seen this before, but if annotations is what you are after, my first choice would be https://web.hypothes.is/.

I was looking at hypothesis yesterday. I'm wondering if you can disable the social features?

I don't know, but with some effort you can self-host everything, and then the social features are under your control.