https://sn.1w6.org/file/clacke-20170727T032312-5lfhb9t.html
The 1.8 vendoring went in at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=746005#278 and it seems that the issues migrating to 2.0 were not language-related any more, but that #lilypond probably uses more of the #guile API than any other software and exposed several issues with it. It also seems that this is being worked on (module time available to the participants) and should be fixed at some point, maybe already within this decade. :-)