Tag for unreleased songs/medleys

I understand the title may be counterintuitive (how can one index a work if the song is unreleased?) but I know of one original medley collab whose medley is not released. I'm using "unreleased" and "not publicly available" synonymously.

In the description of イケメン合作～栄光のイケメドレー～【狩野英孝】, it says that the medley will be released soon, but it still has not been released after 3 years, and if I remember correctly, the host no longer has the intention to release the medley. Moreover, according to w29612's wiki, jok (rickster) is another song that is unreleased. These are not original songs since former is a medley and the latter was not made specifically for the work.

I think there should be a tag or song attribute to note this phenomenon of works using songs that are not publicly available and were shared in private.

I would like to hear some thoughts on this since I'm not 100% sure which way is best to note this phenomenon. Moreover, I am not sure if two items should be created, one for songs and the other for medleys, or if it should all fall under one item.