When you search the internet on how to use net-snmp with IPv6 most of the examples you’ll find look like this:

snmpwalk -v2c -c lab udp6:[3fff:172:20:20::5]  | head -1
iso.3.6.1.2.1.1.1.0 = STRING: "Arista Networks EOS version 4.35.0F-44178984.4350F (engineering build) running on an Arista cEOSLab"

TIL: Apparently since version 5.7 you can just use

snmpwalk -v2c -c lab 3fff:172:20:20::5  | head -1
iso.3.6.1.2.1.1.1.0 = STRING: "Arista Networks EOS version 4.35.0F-44178984.4350F (engineering build) running on an Arista cEOSLab"