If you delete all the LandscapeStreamingProxies under the landscape object the parent landscape object can be deleted.
Not a regression checked in Version: 5.1.0-23058290+++UE5+Release-5.1
Result: You cannot delete it
Expected Result: You can delete it
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-174029 in the post.