From the author: “They have a really high bar”, but are accepted, occasionally: https://news.ycombinator.com/item?id=34480732
> In order to keep SQLite completely free and unencumbered by copyright, the project does not accept patches. If you would like to suggest a change and you include a patch as a proof-of-concept, that would be great. However, please do not be offended if we rewrite your patch from scratch.
SQLite is open-source, meaning that you can make as many copies of it as you want and do whatever you want with those copies, without limitation. But SQLite is not open-contribution. In order to keep SQLite in the public domain and ensure that the code does not become contaminated with proprietary or licensed content, the project does not accept patches from people who have not submitted an affidavit dedicating their contribution into the public domain.
All of the code in SQLite is original, having been written specifically for use by SQLite. No code has been copied from unknown sources on the internet.
It's a strange combination in the free software world, but I'm grateful for it.