There are many closed source applications that have been removed from the Google Play catalog for various reasons.
Often this reason is the cessation of further development and maintenance of the application.
Why don’t these people make the code open to everyone if they don’t need it anymore?
This could bring great benefits to all people, but everything that was once created simply disappears.
It’s probably unmaintained because they don’t care any more, or don’t have the time. If they don’t care or have time, they are not going to spend the effort and time required to open source it and publicise that fact