In release notes of Azure CLI, I see references to "track2 SDK", which I don't understand. When I tried googling, I found Azure SDK API review process, which mentions that only track 2 libraries are reviewed by architecture board, but that does not help in understanding what track 2 is.
Track2 corresponds to the major revamp of Azure SDK client libraries. One of the goal for such major revamp is to improve the consistency among different languages SDK via adhering to guidelines outlined in Azure SDKs Guidelines
The majority of Azure SDKs is auto-generated by Autorest, Track2 generally corresponds to v3+ of each language autorest generator, with generated code being vastly different to previously generated with v2 generators. This will generally require migration, once Track2 SDK updates land, each SDK should provide its own migration guide like servicebus/migration-guide.md
Related: Track 2 Migration Guidance