We're talking here purely about original master tapes held by Columbia in the US. In 1992, five years after the remix for CD, Sony in Holland put out a fine-sounding new pressing of the revised 1966 mix (Columbia 66012 1); it seems a shame that their tape, albeit an at least second generation copy, has never been used for a digital reissue of the album. Presumably overseas subsidiaries are these days controlled more strictly from the centre.
Clearly Sony in the US do still have usable tapes for some of the album's tracks; this is evidenced by fairly recent reissues of Bob Dylan's Greatest Hits and Greatest Hits Vol.II on the Audio Fidelity label. The former contains "Rainy Day Women # 12 & 35", "I Want You" and "Just Like A Woman", while the latter has "Stuck Inside Of Mobile With The Memphis Blues Again". All of these have clearly been remastered from good quality tapes of 1966 mixes. In the case of "I Want You" it is the revised mix that appeared on post-1968 US stereo albums rather than the rejected mix that appeared on early pressings. Of course, it would have been possible for these reissues to have been remastered from the compilation tapes assembled at the time the Greatest Hits records were first produced; but remastering engineer Steve Hoffman has stated on his web forum that for all tracks that were first released on one of Dylan's albums, he went back to the original album masters rather than using the compilation tapes. So whatever tape-wear defects have prevented the release of a complete Blonde On Blonde from the stereo vinyl mixes, these must be on one or more of the songs not listed above.