Is there an official explanation somewhere? I understand that it's maybe not maintained enough but what is the exact reason? Did something stop working? What are the parameters by which a project is deemed not maintained?
easystroke for example is unmaintained for 6 years and crashes Xorg randomly on 18.04 but it is still available and I'm very glad it is.
And amarok has active contributions and I'm not aware of any issue running it on disco.
All applications that do not use the Qt 5 toolkit are being removed from Ubuntu (and Debian).
See https://wiki.debian.org/Qt4Removal (contains a list of qt4 packages and their status in relation to qt5) and specific to Amarok there is a bug report https://bugs.launchpad.net/ubuntu/+source/amarok/+bug/1757590 about removing it.