As already mentioned here:
https://forum.figma.com/archive-21/make-figma-free-for-open-source-projects-21667

It would be awesome to have an open source plan with more features — like unlimited public design files or access to some of the technical features from the Professional plan.

Maybe a good compromise could be a discounted plan — like $10/month — that includes variables by API and similar tools.
For open source developers working in their free time, the $20/month Professional plan is already quite a stretch — and $90/month for the Organization plan is simply out of reach. ‍

All of this is just for public design files anyway — which helps promote Figma itself.
For private projects, I totally understand the need for stricter pricing.