Here is the answer from ChatGPT. Maybe I should create a new user called "ChatGPT" and write a plugin to let ChatGPT help out? 
It is possible to customize the behavior of individual keys on your Mac keyboard, but it will require using the macOS software development kit (SDK). The SDK includes tools and resources for developing custom keyboard layouts that you can use to change the behavior of individual keys.
To create a custom keyboard layout, you will need to use the Keyboard Layout Editor included in the macOS SDK. This editor allows you to create a new keyboard layout by modifying an existing layout or starting from scratch. You can then specify the desired behavior for each key on the keyboard, including the backslash key.
Once you have created your custom keyboard layout, you can install it on your Mac and switch between it and your other keyboard layouts as needed. You can also set your custom layout as the default layout if you want it to be used whenever you are using a particular program or set of programs.
To learn more about creating custom keyboard layouts on macOS, you can refer to the documentation provided in the macOS SDK. This will provide more detailed instructions on using the Keyboard Layout Editor and installing your custom layout on your Mac.