"Myth," Gorbachev said in 2014 interview to ZDF. He added that there is nothing in writing because there was no agreement. Had there been an agreement, they would've written it down. Clip: https://twitter.com/splendid_pete/status/1650735533826375680
And as a bonus feature, in the same clip, USSR's Minister of Defense (1987-1991) says that he's never even heard about it. USSR's Minister of Foreign Affairs gave a similar interview in 2014 too and also called the story BS, adding that he finds nothing wrong with the way Eastern Europe joined NATO.
So there's no proper agreement, just a verbal one, something those that came after could easily ignore, assuming they were even aware about it. And we have to take their word for it because there's no proof. Very smart!
I can point you to the Budapest Memorandum and explain how Russia broke it and why countries like the UK and US have to support Ukraine because 1) the points are very easy to understand and 2) they all signed it.
In any case, one very important question remains. Why do countries want to join NATO? You blame everyone but Russia, but what do you expect to happen when everyone around them has "russians" in need of "protection", their media explains how these small countries could be easily invaded or nuked, Putin starts babbling about what happened 500 years ago and Peter the Great, and so on?
Russia is a pain in the ass for everyone that is unfortunate to have a border with them. From Finland and Ukraine to Kazakhstan and China, they have problems with everyone. Their own behaviour is their main enemy.
Are you able to find that agreement about NATO expansion or not? Can you explain why countries want to join NATO so bad?