They may, at this point, go ahead and remove "get involved" block from their website https://ladybird.org/, since it's not possible to contribute anymore.
> Source-available software is software released through a source code distribution model that includes arrangements where the source can be viewed, and in some cases modified, but without necessarily meeting the criteria to be called open-source.
https://en.wikipedia.org/wiki/Source-available_software
> Open-source software (OSS) is computer software that is released under a license in which the copyright holder grants users the rights to use, study, change, and distribute the software and its source code to anyone and for any purpose. Open-source software may be developed in a collaborative, public manner.
https://en.wikipedia.org/wiki/Open-source_software
And as said here, SQLite was operating like this forever.