While not actually open source, the licensing terms are still quite friendly and still far better than some other much more closed licensing like with Unity, Unreal and Game Maker but not as open as something like Godot. Defold is progressing on though and their roadmap for 2021 mentions their plan to continue to improve their Linux support.
Under the Platforms heading on their roadmap was this:
Editor on Linux
The Defold editor runs well on Linux but there are a few pain points and problems related to Linux distributions, window managers and graphics drivers. This year we plan to invest time in order to fix at least some of these issues.
Nice to see their developers are not only paying attention to reports but actually want to do better. Their roadmap is also up on GitHub so anyone interested can directly follow various support tickets and feature requests to see how it's doing.
Much more changes are planned too like pulling out their 2D skeletal animation Spine code and the Box 2D / Bullet physics systems and putting them into extensions. Currently both are tightly integrated and limited in how they can push them further, so having them as extensions will allow a lot more flexibility for both game developers and the people working on Defold directly.
They're also working on multiple features to improve performance of games built with Defold like object culling which developers need to do manually right now, improved texture compression, reducing draw calls for sprites and much more.
Sounds like 2021 is going to be big for Defold.