NavMeshAgent.currentOffMeshLinkData.offMeshLink returns a reference only to an OffMeshLink component, and not to a NavMeshLink component.

When the value of the NavMeshAgent.isOnOffMeshLink property is true but NavMeshAgent.currentOffMeshLinkData.offMeshLink is null, this indicates that the link is either a NavMeshLink or a type of Off-mesh link that has been automatically generated. In that situation the property `NavMeshAgent.navMeshOwner` can point to the component that has created the link, if that was the case.

Here is an example of code that retrieves the area type of the link that the agent traverses:
```
var _agent = GetComponent<NavMeshAgent>();
var linkAreaType = 2; // 2 is the Jump area used by all auto-generated links
if (_agent.currentOffMeshLinkData.offMeshLink != null)
{
linkAreaType = _agent.currentOffMeshLinkData.offMeshLink.area;
}
else
{
linkAreaType = _agent.navMeshOwner switch
{
NavMeshLink navMeshLink => navMeshLink.area,
OffMeshLink offMeshLink => offMeshLink.area,
_ => linkAreaType
};
}
```

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Side note: Starting with 2023.2 the newly-added NavMeshAgent.currentOffMeshLinkData.owner property references the NavMeshLink or the OffMeshLink that has created the navigation link.
https://docs.unity3d.com/2023.2/Documentation/ScriptReference/AI.OffMeshLinkData-owner.html