The Extreme Networks MIBs are located at www.extremenetworks.com/support/policies/mibs/.
You need to provide your serial number or agreement number, and then the MIBs are available under each release.
For detailed information on which MIBs and SNMP traps are supported, see the Extreme Networks Proprietary MIBs and MIB Support Details sections in the ExtremeXOS 22.5 User Guide.