In this post we can learn that Attributes from JetBrains.Annotations are embedded in UnityEngine.dll.
In this bug report we can learn that the embedded attributes are not up to date with the newer versions of the nuget package.
Will we ever see an update to the embedded JetBrains.Annotations? If not, could Unity provide an easy upgrade solution to this problem? Simply adding the JetBrains dll is not possible due to type conflicts that come from the embedded version of the types.