I never looked into the guts of how this was implemented, but I worked on a product which had an SNMPv3 agent that was only restricted by a username and password. I could flash a PC with a fresh Ubuntu image, apt install Net-SNMP and start sending SNMPv3 requests without every futzing with any keys.
If I remember right, handling SNMPv3 traps required some messy key stuff so the agent still sent SNMPv2 traps, but there was no requirement for keys for GET/SET.
If I remember right, handling SNMPv3 traps required some messy key stuff so the agent still sent SNMPv2 traps, but there was no requirement for keys for GET/SET.