You're wrong. There was indeed never anything "official", but high-level diplomats (such as the US Secretary of State) did promise as much behind closed doors. See https://nsarchive.gwu.edu/briefing-book/russia-programs/2017...

They were informal assurances. Jeffrey Sachs was there at the time. https://www.jeffsachs.org/newspaper-articles/gafr872ellaha3w...