The new repository can be found over at: https://github.com/SolidOS/solid-ui.
This repository ONLY contains icons. These icons were used in pre 1.8.2 mashlib and pre 1.4.16 solid-ui.
In solid-ui we had the icon base uri pointing to https://solid.github.io/solid-ui/src/. With the move of the SolidOS GitHub to a new organisation (SolidOS) this link changed. To fix the problem, in general, we have an open ticket: SolidOS/solid-ui#496.
Till further notice, for pre 1.8.2 mashlib and pre 1.4.16 solid-ui versions, this repos is in place.