Currently SNMP OID field limit stands at 512 characters.
Due to walk item being implemented, the OID list can get quite huge.

For example, VMWare/VeloCloud SD-WAN Edges (https://mibs.observium.org/mib/VELOCLOUD-EDGE-MIB/). As you can see, there’s a lot of OID’s in that table. Zabbix however is truncating the item key to 512 characters which is insufficient to walk all of the OID’s in that table.