All Android devices have an icon in the status bar if there's no SIM installed
which is annoying. You just need to keep all those old SIMs from when you
change telcos and change SIM sizes.

Well, if you don't like the [!] icon in the notification tray, you can switch it for an aeroplane :) ... putting a phone in flight mode should remove the "Emergency calls only" warning, and on most handsets I've seen, you can toggle the WiFi and/or Bluetooth back on after going into flight mode. Will also prevent phone asking for SIM pin, if SIM (inactive or no) has one.