There's a disturbing trend that's similar to Microsoft's Shared Source [1] initiative from last decade, used by GitLab Enterprise [2] and Greensock [3]. The code is available for anyone to download, but is not under an Open Source license. They have the option of accusing their competitors of looking at their source code. [1] https://en.wikipedia.org/wiki/Shared_source [2] https://gitlab.com/gitlab-org/gitlab-ee/ [3]…
It's useful to note that the term "open source" does not require an OSI-approved license. It is conventional that the term implies an OSI license, but there are projects that are undeniably open source which are not issued under an OSI license. SQLite being one of the most prominent ones. All that can be said today formally about a project is whether or not it's OSI-licensed, rather than whether it's open source. So…
As for SQLite, it's compatible with OSI licenses. It has been folded into a larger work that's licensed under an OSI approved license countless times. There are some projects that might throw around the term public domain but I'd have to examine the details to see if it's ready to be used as part of an open source project.