We can all agree it's best practice to implement a SOC within an organization, but are there any frameworks or compliance requirements that specifically call it out?
In the PCI DSS, section 12.5.2 calls out the requirement to assign responsibilities to a person or team to "Monitor and analyze security alerts and information, and distribute to appropriate personnel."
It may not be spelled 'SOC', but it matches the definition.
I disagree with your premise.
"Monitoring" is required by many frameworks and regulatory requirements, but I'm not sure there is value in specifying the overhead of a SOC to implement monitoring so specifically.
Setting up a SOC is a natural consequence of being compliant with the various frameworks. In order to be consistent, to have staff on hand to respond, and having repeatable and consistent processes for monitoring and response can be done in a variety of different ways. Once the number of things to monitor, the number of events, and the number of incidents reaches a critical point, the natural next step is setting up a SOC.