The MiG-21 currently only uses a LOD0 model. It has a .lods file describing transitions but they all reference the LOD0 model and the LOD0 model has no embedded transform nodes.
While the model is not the heaviest in DCS, in a multiplayer environment near a busy airfield, a few MiG-21s can add up to a lot of objects and triangles very quickly, putting an unnecessary load on the player's CPU & GPU, particularly with shadows enabled. Also, due to how the camera seems to work in DCS, zooming in can cause the 3D model to begin rendering at longer distances.
I did a search in this forum for the term "lod" and saw references to different LOD levels in the past, so possibly they did exist once and have been omitted along the way.
Attached are two in-game screenshots showing 10 MiG-21s taking off at Senaki airbase, at close range and 6km away, with similar object counts in both scenes.
I hope you can find the time to add the lower detail LODs as it would be a great help to the multiplayer community, especially the Cold War servers.