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 ("living room bookcase"). That way if someone (including yourself) years from now looks at the tangle of wiring and wonders "Hmmm, maybe this wire might go where I need it" or "What does this do, can I remove it?", 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.