[wix] How to add a WiX custom action that happens only on uninstall (via MSI)?

There are multiple problems with yaluna's answer, also property names are case sensitive, Installed is the correct spelling (INSTALLED will not work). The table above should've been this:

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Also assuming a full repair & uninstall the actual values of properties could be:

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The WiX Expression Syntax documentation says:

In these expressions, you can use property names (remember that they are case sensitive).

The properties are documented at the Windows Installer Guide (e.g. Installed)

EDIT: Small correction to the first table; evidently "Uninstall" can also happen with just REMOVE being True.