Polygon Reducer optimizes your meshes by reducing their vertex count in a way that keeps the visuals as close to the original as possible. It works great for setting up levels of detail (LODs)!

Polygon Reducer makes it easy to optimize your game – simply drag the reduction slider and watch your mesh be transformed in real time. It works both at runtime and in the editor!

Supports:

All Long Term Support (LTS) versions of Unity;
All render pipelines: Built-In, URP, HDRP, and SRP;
Skinned models with animations;
Models with any number of submeshes.

And also:

Preserves all mesh data: UVs, normals, tangents, bone weights, and more;
Has a low memory footprint;
Creates reduced meshes at runtime – no need to store them in your disk!