It is widescreen, according to this…

https://www.facebook.com/warnerarchive/posts/10155716901136563

SUPERMAN: THE MOVIE - EXTENDED CUT & SPECIAL EDITION 2-FILM COLLECTION (1978,2000)
Run Time 188 Minutes-EXTENDED CUT (NEW 2017 1080p HD MASTER)
151 Minutes-SPECIAL EDITION
Subtitles Spanish, French, English SDH
DTS HD-Master Audio 2.0 – English (EXTENDED CUT)
DTS HD-Master Audio 5.1 - English, DOLBY DIGITAL SURROUND 5.1 - French, DOLBY DIGITAL SURROUND 5.1 - Spanish
ORIGINAL ASPECT RATIO - 2.40:1, 16 X 9 LETTERBOX
COLOR
2-BD 50