"Shared source" seems to originate from Microsoft, from the time period before it embraced OSS proper. At least the first time I've seen it in any context was the "shared source CLI", which was released under the look-but-do-not-touch license as a reference implementation to back the ECMA CLI and C# standards.

https://en.wikipedia.org/wiki/Shared_Source_Common_Language_...

That said, it's a surprisingly good term, because it's so accurate - the source is shared with you, but there isn't much you can do with it other than look at it.