Static Mesh LODs cannot be removed after import from the Static Mesh Editor unless you have Simplygon installed. Hovering over the tooltip for "Number of LODs" indicates this as well with "Auto mesh reduction is unavailable! Please provide mesh reduction interface such as Simplygon to use this feature or manually import LOD levels."
As an example, If I import a single mesh and then manually import a LOD using the "LOD Import" drop-down and select "Import LOD Level [X]", I will never be able to remove that LOD without reimporting the base LOD to wipe any existing LODs that were manually imported because the slider is grayed out.
-Ensure that Simplygon has not been configured for the editor
-Add a LOD to a static mesh (easiest to just export a static mesh with only one LOD, and then reimport it as LOD1)
-Attempt to remove LOD1 from the static mesh
Expected result: LOD can be removed
Observed: Impossible to remove LOD.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-25591 in the post.