I figured it would be an interesting topic to make a list of any known commentary tracks that are unreleased for whatever reason. Here are a few that I know of.

Halloween H20
Steve Miner & Jamie Lee Curtis
Printed on the back of the dvd cover, but it's not on the disc. I'm assuming the commentary was recorded, but this is the only proof.

Quantum of Solace
Mark Forester
The director mentioned in 2009 that he had just finished working on a special edition of the film, along with recording a commentary track. It's assumed the release has been held off until the next Bond film hits theaters.

G.I. Jane
Ridley Scott
I'm going off memory here, but in 1997 I remember being in a Suncoast reading a laserdisc pamphlet they used to hand out. Laservision or something like that. It stated that 1998 would see the release of a special edition featuring an extended cut of the film and a commentary track. I can only assume the quick demise of laserdisc in 98 stopped the release.
Still doesn't explain why it didn't end up on dvd.
EDIT: Just found out this track ended up on the laserdisc of the film. It was not a extended cut although the disc did feature an alternant title sequence as a special feature.

Invasion of the Body Snatchers
Joe Dante & Kevin McCarthy
Recorded for a special edition dvd around 2001/2002, it never has been released. Artisan entertainment was producing the dvd when the rights to the republic library reverted back to Paramount.

Scrooged
Richard Donner
SCROOGED: YULE LOVE IT EDITION was announced with artwork in September 2006. Never met release date, and has never been talked about since.

Now is one I am not sure about, but would love input from anyone who knows.

The Crow
Alex Proyas
Proyas took part and recorded a commentary for the 2001 release of the dvd. Something happened and Proyas walked away from the project. Is the new Blu Ray track the one recorded in 2001 or a whole new track recorded recently?

South Park Season 1

One of the Bond movies was released then later recalled or maybe just removed from future releases.

Well, both of these examples ARE released, just either on CD (South Park) or were on the laser discs. (First 3 Bond films)

Susan Orlean's commentary for Adaptation.

I remember hearing way back when that there was a commentary for Fight Club recorded by the Dust Brothers (possibly as part of an isolated score?). It was dropped from the DVD for space reasons, but it didn't get put on the Bluray where it would have fit.

I don't know if these count, but a few commentaries were recorded without any specific plans to get put on DVD, such as the ones here for Halloween 6, Child's Play and Fright Night:
http://iconsoffright.com/pirate-commentaries-2/
And here's a fan one for Ladies and Gentlement, the Fabulous Stains that was recorded in hopes of getting on the DVD but was left off
http://home.earthlink.net/~meheuck/thefabulousstains/

Alex Proyas' original commentary is on the newly released blu-ray, US edition. And thanks for posting the Halloween 6 commentary. Great thread

No unreleased but very hard to find: I would love to hear the commentary track the League of Gentlemen did for Blood on Satan's Claw. It's only on the (way out of print) Trigon box set from Anchor Bay UK.

Natalie Portman apparently did a commentary for the 2 disc Superbit version of Leon, but its nowhere to be found

that's interesting about Leon & Portman.

I'm curious about the new Crow commentary as well.

Darren Aronofsky for "THe Fountain", which is available on his website for streaming.

That's an urban legend and never been proven.

That really is a great commentary, I really enjoy that box set.

:shakesfist:

I heard that RIchard Matheson did a commentary for MASTER OF THE WORLD, back when MGM was doing it's Midnight Movies series. MGM dropped the line and ultimately released MOW as a MOD disc, but there is no commentary.

I just listened to the Halloween 6 commentary…very interesting indeed. Farrands really expresses his disappointments with the film…specifically about how Dimension wanted the more gory approach to appeal to teens, and wanted to cut most of Donald Pleasance’s part. Also talked quite a bit about Paul Rudd’s mindspace during filming, and how today he is actually proud that he was involved in a Halloween film. One of the better commentary’s that I’ve heard in a long time.

Oh, another.

Mario Bava's "Kill Baby Kill" commentary by Tim Lucas, from the recalled Dark Sky DVD.

I think that sometimes actors confuse recording a commentary with being interviewed for a documentary. Bruce Campbell did it earlier this year with Evil Dead 2. I know that other actors have done this too, but I can't think of any off hand.

Found a few more.

The Terminator
Lance Hendrikson and Gale Ann Hurd
Circa 2009, Hendrikson mentioned the track in an interview. It was later stated on the HTF that the track would be mixed with several other tracks recorded. It was not said what other participants were to be included. Evidently a full blown updated special edition was planned for 2010, but was put on hold for some reason.

Apt Pupil
Bryan Singer
Around 5-6 years ago when Sony controlled the home video rights to the MGM libary a box set containing a new S.E. of Usual Suspects, Apt Pupil and a 3rd disc featuring Bryan's shorts and early work was being prepped. Singer was supposed to record a new commentary track for AP, there is no proof that it was ever recorded though.

Harrison Ford did this with the Indiana Jones DVD's as well - he said in an interview that he "did commentary" for them, when he just meant that he was interviewed for the documentaries.

On the subject of The Evil Dead movies, on The Ultimate Edition special features that focused on the 3 actresses, one mentions that they recorded commentary, but that's nowhere to be heard (though it might be another case of confusing being interviewed with "commentary").

G.I. Jane
Ridley Scott

EDIT: Just found out this track ended up on the laserdisc of the film. It was not a extended cut although the disc did feature an alternant title sequence as a special feature.