This is a paid asset, but now you can download AutoLOD – Mesh Decimator for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
AutoLOD – Mesh Decimator v3.3.0
AutoLOD is a powerful tool that will setup a LODGroup with any mesh from your scene. It will automatically compute simplified versions of your meshes using decimation.
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Minimalist and really easy to use
Works in both edit and play mode
Works on prefabs instances (no need to unpack them)
Compatible with Skinned mesh renderers too
Generates up to 8 LOD for a single object
Able to decimate the UVs (UV1 to UV4)
Compatible with submeshes
Has a balance option between high performance and high-fidelity result
Can be used for simple distance culling
Uses the ‘Fast Quadric’ mesh decimation algorithm.
Works on any render pipeline
UI based on the native UnityEditor library only.