There was a full frame version released on Laserdisc.
http://www.lddb.com/laserdisc/24026/LV32301/Star-Trek-VI:-The-Undiscovered-CountryTUC was among the very few Paramount films released on Super VHS in the early 90’s, but the packaging is almost impossible to tell apart from the regular VHS version save for the format logo on the box.
Oh wow, good find.
Hope someone can preserve this.
You could always purchase the Fullscreen VHS. It goes fairly cheap on eBay.