Well, with the release of the 75th Anniversary edition of "The Wizard of Oz" (now available in (fake) 3D), it's time to present what little I know about the availability of the original mono:

VHS: Unknown if VHS had original mono. (Can somebody verify?)

LD: Some LD's are mono (like the 50th Anniversary Edition), others were fake stereo (like a Japanese release with reference number G98F5545). As far as I know, it is original mono. (Can somebody verify?)

DVD: Dolby Digital 5.1 for the simple release, mono included in the Collector's Edition and 70th Anniversary Edition. The mono track is slightly altered.

BD: Both Dolby TrueHD 5.1 and mono sound is available on the 70th Anniversary Blu-ray. Mono is unavailable on the 75th Anniversary Blu-Ray. Mono is altered.