I also doubt the claim by Shanks that Zakarpattia Oblast, to which
Uzhgorod / Uzhhorod belongs, had Central European time between 1 July 1990 and 31 March 1991.

Zakarpattia Oblast has borders with Slovakia (was Czechoslovakia still in 1990) and Hungary.
These countries were on Central European time (GMT +1h east) during that period.
The town of Uzhhorod is directly at the border to Slovakia.

Kiev, Ukraine's capital, was under GMT +3h east at that time.

Under which authority the more western Zakarpattia Oblast would have followed its own time zone?

I have put the question now into an Ukainian astrology forum, but my experiences with this kind of forum are
not the best - few people outside the small TZ community engage themselves with details of historical time zones.

The Ukrainian astrology website on which that forum runs uses the time zone of Kiev for Zakarpattia Oblast, during the period in question.

I am trying to find out on which information that is based.

On 11/26/19 6:04 AM, Mikhail Gribanov wrote:

Europe/Uzhgorod
Europe/Zaporozhye

Can you explain why they were added? I did not find any evidence of
the existence of events "Ruthenia used CET 1990/1991" (comment in tz
database).

As I recall, they were added because Shanks's International Atlas had them. The "europe" file says:

# Unless otherwise specified, the source for data through 1990 is:
# Thomas G. Shanks and Rique Pottenger, The International Atlas (6th edition),
# San Diego: ACS Publications, Inc. (2003).
# Unfortunately this book contains many errors and cites no sources.

It's quite possible that Shanks was wrong, or that I transcribed Shanks incorrectly. If so, it'd be helpful to provide evidence of the error, so that we can fix tzdb.