There’s a Kickstarter running right now for a free and open-source Sketch or Figma alternative called Akira: https://www.kickstarter.com/projects/alecaddd/akira-the-linux-design-tool . There’s nothing quite like it available in the open-source world or for Linux machines, and it’s a great alternative to Figma if you don’t want to be spied on.
Unlike Inkscape, which is a well-known open-source vector editor, Akira would be designed for UX and UI design, just like Sketch. That means having artboards for creating multiple screens or multiple versions of an icon, efficient Sketch-like UI, pixel-perfect alignment, and being able to manage layers easily. There is also discussion of being able to export Akira files into SpiceUp (a presentation editor) for quick prototyping.
If you’re a designer or even just an open-source enthusiast, please chip in or at least help spread the word.