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Blender, Firefox and OpenOffice have, from the top of my head.

Blender didn't start as opensource project.

That's what the person you're replying to is asserting as well.

Firefox is debatable. Netscape 6 which it descended from was a total rewrite and never really had much/any closed source release history to it (unlike earlier Netscape releases)

Well, there's a lot - a few I happen to be familiar with:

Maxima: https://maxima.sourceforge.io/

Axiom: http://www.axiom-developer.org/

OpenCascade: https://dev.opencascade.org/

BRL-CAD: https://brlcad.org/

GRASS: https://grass.osgeo.org/about/history/

illumos: https://illumos.org/

Docker: https://github.com/docker

Torque: https://github.com/GarageGames/Torque3D

Better than just dying and taking the source with it.

Synfig Animation Studio