Author here. I will take the opportunity to advertise my blog at https://fredrikj.net/blog/ where I post occasional development updates about Arb and other projects.

Some improvements not covered in the 2016 preprint include much faster code for arithmetic and matrix multiplication (https://arxiv.org/abs/1901.04289, https://fredrikj.net/blog/2018/07/high-precision-linear-alge...), and code for rigorous arbitrary-precision numerical integration (https://arxiv.org/abs/1802.07942, http://fredrikj.net/blog/2017/11/new-rigorous-numerical-inte...).