Works in Unity 4, 5, 2017, 2018, 2019 and 2020.

Old Mouse Methods is a fix for those who want to both use Unity's new input system and keep the MonoBehaviour mouse methods working (OnMouseEnter, OnMouseOver, etc)

Plugin website | Documentation | Developer website

The Problem
Unity's new input system is great. It takes input management to a whole new level. However one of the system's known limitations is to break a convenient feature, the MonoBehaviour mouse methods:
•OnMouseEnter
•OnMouseOver
•OnMouseExit
•OnMouseDown
•OnMouseDrag
•OnMouseUp

If you switch to the new input system package and have any of these methods in your scripts, they will never be called.

The Solution
This plugin makes the mouse methods work again.

How To
Just add our C# component script once, to any object in your hierarchy, and then the mouse methods will be available in all your objects.

Whether you are starting a new project from scratch with the new input system, or migrate an existing project to the new input system, this plugin allows you to have the best of both worlds.