My objection is about Vulkan specifically, not about using the GPU. It makes this software less portable than if they had used OpenGL, and for no benefit that I can see.

I mean, your original comment specifically cited a software renderer, and your original objection was that vulkan was overkill, not that it wasn't well supported, so it seems like you're moving the goalposts all over the place but I'll bite.

As someone who does graphics programming, I would not choose OpenGL for new projects. For one thing it's the future: while I only expect Vulkan to become more well supported as time goes on, the opposite is happening with OpenGL (see the state of support on Apple platforms). Also the Vulkan API itself actually makes it easier to target more platforms. Because the OpenGL api takes on more responsibility over how the graphics pipeline is executed, there is actually a lot more variation in behavior across driver implementations than you have with Vulkan. This was one of the biggest headaches of OpenGL development which Vulkan has largely solved.

So if I had to choose between losing some users who are using old hardware verses the headaches of supporting an OpenGL application across platforms, along with the fact that OpenGL is actually getting less support over time, the choice would be easy for me.

It's a moot point anyway since the developer here is already using a high-level compatibility library.

rx uses wgpu-rs, which primarily supports Vulkan, Metal, and DX12 (though DX11 and GL backends are also partially supported). This is aready relatively portable compared to using GL directly, especially on macOS where it has been deprecated.