You can use OpenUPM to install UnityMeshSimplifier.
Follow these steps for getting the OpenUPM CLI installed.
Go to the project directory in your favorite terminal.
Type openupm add com.whinarn.unitymeshsimplifier in your terminal to install UnityMeshSimplifier.
openupm add com.whinarn.unitymeshsimplifier
Success! Start up Unity with your Unity project and you should see UnityMeshSimplifier appear in the Unity Package Manager.
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