Unissued recordings that eventually appeared or unissued recordings that have still to appear?
The second category is, by its very nature difficult to quantify but I can offer one. When Simon Rattle and the CBSO performed the Mahler 7 at the Proms in 1989 I went backstage afterwards and asked SR if they were recording it. He told me that they were taking it into the studio next day. For whatever reason, though I believe it was because the Prom account proved impossible to capture in the studio, it was never issued. A live account appeared in 1991. EMI should therefore have an unissued Mahler 7 in their vaults.
“Every piece of music is a rehearsal of one’s life,” - Sir Colin Davis