Ref field declarations should require -langversion:11 or later and the runtime feature System.Runtime.CompilerServices.RuntimeFeature.ByRefFields.

Currently, the compiler is only checking language version.

From the spec:

This feature requires runtime support and changes to the ECMA spec. As such these will only be enabled when the corresponding feature flag is set in corelib. The issue tracking the exact API is tracked here dotnet/runtime#64165

See #62304 (comment)

Relates to #59194