You can definitely take the spirit of the 8pt grid into any numerical aspect of your design system.

Typography, opacity, whatever!

I have just started using it for border radii. I use 0, 4, 8, 16, 32 for border radii depending on the size of the component. (The larger, the bigger the border radius.)

I also work primarily on a template that is used by other designers. So an added benefit of the 8pt grid is that when a designer sees something that has an "odd" number, they will ask if it's a mistake and it keeps things moving much faster and cleaner.