Each major, minor, and patch release of ArcGIS Pro includes fixes for bugs that were reported to Esri Technical Support. The bugs that were fixed are listed in issues addressed lists.
See the Issues Addressed list for 2.9.0 and 2.9.1 [PDF].
More information about an issue on the list can be found by searching for the BUG number on the Technical Support site.
Major and minor releases also include new functionality, which is described in the what's new topic.
ArcGIS acknowledgements
The acknowledgements for ArcGIS can be found on Esri's legal website.