Additionally some BD releases of Halloween and Mad Max have mono tracks that are actually downmixes of the surround track. The recent UHD release of Halloween sadly is one of them.
Yeah, don’t be fooled by studios simply claiming that the original tracks are included. Superman: The Movie is one of those where the first few BR releases claimed to have the theatrical stereo when it was really a fold-down of the modern remix.
Then there’s also the fact that a lot of times, when original mixes are provided, they’re in much lower quality (say, 192kbps AC3) compared to the modern remix (often lossless). Older laserdisc releases’ uncompressed PCM audio will often be superior, even if it’s the same mix.
I actually did do some deep research into this topic and here’s what I’ve figured out. The 2000 DVD release that only has the extended version is where the 5.1 remix first surfaced, then finally an anthology DVD set in 2006 included both versions and the theatrical version disk was supposed to also include the original 2.0 mix but was a down fold like you said earlier, so this led to Warner Home Video issuing corrected disks with the right 2.0 mix. The 2006 Blu-ray release once again only contains the extended version with only a 5.1 remix provided. In 2011 an Anthology Blu-ray was issued that once again brings all the movies together and both versions of Superman 1 and 2 were included. The disk for the theatrical version of the original Superman includes both a 5.1 remix and a 2.0 mix (both encoded in a lossless DTS-HD form). Warner Home Video swore that the 2.0 mix was the original theatrical mix and even says “English 2.0 (original theatrical)” if you go to the audio choices on the main menu of the theatrical version disk. Finally when the recent 4K release happened, Warner went back to the original elements by scanning the original negative in 4K, and making a new Dolby Atmos soundtrack that Warner took from the original sound elements which supposedly ignores the 5.1 remix from 2000.