Quant-UX is worth a look for prototyping needs, it's OSS as well and can generate end HTML/CSS code: https://quant-ux.com/

Lunacy supports multi-player in the way Figma users are used to: https://icons8.com/lunacy

Frame / Plasmic generates code, the latter focusing on React: https://www.framer.com / https://www.plasmic.app/

But yes it does appear that Penpot for now is the most obvious de facto Figma alternative.

The silver lining of this whole debacle has been that it's brought to surface a lot of other alternatives that folks would not have otherwise considered or looked into, which are actually quite decent I'm finding out.