I've been thinking about this a lot lately, as I recently started a project where, since I'm not a designer, I'm trying to get designers onboard to help out with everything design. So far we had some people, which is nice, shows that there is willingness in designers to do open source work. Hardest so far is for me as a non-designer to understand the best workflows for designers to collaborate on design. Figma helps us a lot to collaborate, but it's just one piece of the puzzle. I'm sure there is more we can do to facilitate designers to help out with open source projects, we just have to find a "send patch / pull request" workflow that works for designers, just as we did for developers. Once we have some workflow nailed down, it can hopefully be replicated to other open source communities in order to have more designers collaborate with developers on open source.