The only missing multi-track masters that I know of are from "In Search Of The Lost Chord". They would have been done back in the Quad era if they could have been found. Were "Days Of Future Passed" also lost at a later date?
I don't think the "Days of Future Passed" tapes were ever lost, but they suffered some serious degradation issues, so they remixed the album in the 1970s in both stereo & Quadraphonic while the tapes were still in stable and usable condition.
I do think it's cool that they were able to recover and remaster the original stereo mix for last year's 50th anniversary reissue, though in terms of sound quality, the 1970s remix is much better.