"[Swift] was initially a proprietary language, but version 2.2 was made open-source software under the Apache License 2.0 on December 3, 2015"
Darwin is also a bad example:
"On July 25, 2006, the OpenDarwin team announced that the project was shutting down, as they felt OpenDarwin had "become a mere hosting facility for Mac OS X related projects", and that the efforts to create a standalone Darwin operating system had failed.[40] They also state: "Availability of sources, interaction with Apple representatives, difficulty building and tracking sources, and a lack of interest from the community have all contributed to this."[41]"
"PureDarwin is a project to create a bootable operating system image from Apple's released source code for Darwin.[43] Since the halt of OpenDarwin and the release of bootable images since Darwin 8.x, it has been increasingly difficult to create a full operating system as many components became closed source."