“So, we can believe this dubious source or declassified intelligence documents that say we promised Russia no eastward expansion of NATO”\
1. We “promised” RUSSIA nothing. All the DISCUSSIONS you refer to were (a) in a cold war context, (b) with western leaders and THE SOVIET UNION, not “Russia”, and (b) relative to relationships with that still existent SOVIET UNION and a still existent Warsaw Pact.
2. As the Soviet Union collapsed is was hoped by western leaders that Russia would liberalize, no longer be a dictatorship and no longer take a cold war view of the west. That the west would be able to relax more vis-a-vis Russia. THAT DID NOT HAAPPEN.
Even your own left-wing and only source denies that.
And, declassified documents prove it.