All Company Classic community used to be a "phantom" community in Yammer that contained the messages in the All Company feed. In early 2022, Yammer converted the “All Company” feed into an actual connected community to support Native Mode. All Company now works like other Yammer communities - Yammer | Microsoft Learn
Most messages were transferred in the backend to the new connected community (the one with the number in square brackets in tyGraph reports) by the Yammer team, but some may have remained attached to the old “phantom” community. These may be some very old or deleted messages.
Yammer has recently released some new features, Stories and Storylines, that are now being associated to the old "phantom" All Company feed community. If you are seeing new messages being associated to "All Company Classic", that means you are looking at Stories and Storylines. tyGraph is actively working on delineating Stories and Storylines to provide a new reporting experience around these new features. Stay tuned!