It's been closed source for 16+ years. Some background:

* https://blog.paint.net/2007/12/04/freeware-authors-beware-of...

* https://blog.paint.net/2009/11/06/a-new-license-for-paintnet...

Paint.Net was only really open source entirely from 2004-2007 and partially from 2007-2009 (engine only, not the UI for the latter part).

My experience: Pinta is an inferior Paint.NET . It's about the same level of complexity, but everything is just like... 60-85% as good.

Krita is a heavier program, but has a similar level of polish. You might not use half the features, but Krita's what I'd suggest for anyone wanting an open source alternative to Paint.net

If open source matters more than anything else, there's always GIMP.

If you just want to use the thing, not recompile it, paint.net is still free, even for commercial work, and (in my opinion) has better UX.

Pinta is based on an old Paint.net code

Pinta is obviously not as capable, but yeah, it's by far the most usable general-purpose photo editor on Linux that I've found.

If you really miss the simplicity of MSPaint and need something that's quick and no layer complications: KolourPaint.