We define FeatureCompatiblity here. However, there are some immediate downsides:

• Checks that reference the version need to write out ServerGlobalParams::FeatureCompatibility instead of simply FeatureCompatiblity.
• mutableFeatureCompatibility is directly exposed.
• Our default value does not match our reset value, implying that the state can be accessible but unset. This would be more intuitive if we could provide optionality via either a pointer or an optional.