After I have installed the plugin from the Asset Lib I can see the plugin in the lower left side in the File System tab. Yet when I inspect the Project Settings window, the Plugins tab is empty.
See the screenshot I included, please.
I just looked into this for a bit, the plugin you have mentioned is for 4.0 and 4.1 only, as it says in the description of it in the asset store.
It also mentions you should use “Godot OpenXR Vendors plugin” instead for 4.2.
Additionally the asset for 4.0 and 4.1, like some other assets that are not plugins do not show up in the plugins list as they do not need to be initialized the way plugins do.
For example if you do want to use that specific asset for 4.0 and 4.1 instead of the updated version for 4.2 there is instructions as to how to use it here