Is it permissible to cover over an ethernet (or other low voltage) wall plate
You can block it. However, you should consider one of three other options:

Disconnect - Find the other end. Disconnect it. Label it (&quot;living room bookcase&quot;). That way if someone (including yourself) years from now looks at the tangle of wiring and wonders &quot;Hmmm, maybe this wire might go where I need it&quot; or &quot;What does this do, can I remove it?&quot;, you won't have to go hunting all over the place to figure it out.

WiFi Hotspot - Depending on your house, adding WiFi hotspots may be a useful thing. Since you have an Ethernet connection sitting unused, you could use this as a connection for a WiFi hotspot. If you use a hotspot that gets power over Ethernet then you don't have to worry about electric power here, so it is very safe. You can even route a patch cable (if you don't feel like running the wire farther inside the wall) behind the bookcase and stick the hotspot on top for better coverage.

Convenience Outlet - Depending on location, either right where it is now (cut a hole in the back panel of the bookcase) or higher up (use this as a junction to run to another box installed at a more useful height and cut a hole in the back panel of the bookcase there) or on the side (use this as a junction to run through the bookcase, coming out with a jack installed in one of the side panels). This provides a convenient way to temporarily connect either a non-WiFi device or a WiFi device where you want a more stable connection.