I still get this error in Cosmic on a Dell M6500.

The M6500 does not have a keyboard RFKILL shortcut - in only has a physical switch. That switch correctly enables/disables WiFi.

Is this the same bug?

Dec 18 09:04:28 localhost kernel: [ 2706.037523] dell_wmi: Unknown key with type 0x0011 and code 0xffd0 pressed
Dec 18 09:04:28 localhost kernel: [ 2706.037526] dell_wmi: Unknown key with type 0x0011 and code 0xffd1 pressed
Dec 18 09:04:28 localhost kernel: [ 2706.037527] dell_wmi: Unknown key with type 0x0011 and code 0xffd0 pressed

Note that I never use the Fn-keys, so I have no idea which key(s) it would be complaining about.

$ uname -a
Linux DellLaptop 4.18.0-12-generic #13-Ubuntu SMP Wed Nov 14 15:17:05 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

$ cat os-release 
NAME="Ubuntu"
VERSION="18.10 (Cosmic Cuttlefish)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.10"
VERSION_ID="18.10"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=cosmic
UBUNTU_CODENAME=cosmic

Here is the output from using the physical switch:

Dec 18 11:24:13 localhost kernel: [11090.779775] atkbd serio0: Unknown key pressed (translated set 2, code 0x88 on isa0060/serio0).
Dec 18 11:24:13 localhost kernel: [11090.779781] atkbd serio0: Use 'setkeycodes e008 <keycode>' to make it known.
Dec 18 11:24:13 localhost kernel: [11090.779888] iwlwifi 0000:0c:00.0: RF_KILL bit toggled to disable radio.
Dec 18 11:24:13 localhost kernel: [11090.779899] iwlwifi 0000:0c:00.0: reporting RF_KILL (radio disabled)
Dec 18 11:24:13 localhost kernel: [11090.780076] wlp12s0: deauthenticating from 24:b6:57:f9:c6:9e by local choice (Reason: 3=DEAUTH_LEAVING)
Dec 18 11:24:13 localhost kernel: [11090.780986] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.780991] wlp12s0: HW problem - can not stop rx aggregation for 24:b6:57:f9:c6:9e tid 0
Dec 18 11:24:13 localhost kernel: [11090.781003] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781011] wlp12s0: HW problem - can not stop rx aggregation for 24:b6:57:f9:c6:9e tid 1
Dec 18 11:24:13 localhost kernel: [11090.781019] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781023] wlp12s0: HW problem - can not stop rx aggregation for 24:b6:57:f9:c6:9e tid 2
Dec 18 11:24:13 localhost kernel: [11090.781028] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781031] wlp12s0: HW problem - can not stop rx aggregation for 24:b6:57:f9:c6:9e tid 3
Dec 18 11:24:13 localhost kernel: [11090.781197] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781202] wlp12s0: failed to remove key (0, 24:b6:57:f9:c6:9e) from hardware (-5)
Dec 18 11:24:13 localhost kernel: [11090.781359] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781366] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781378] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781387] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781419] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781433] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781874] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.781882] wlp12s0: failed to remove key (1, ff:ff:ff:ff:ff:ff) from hardware (-5)
Dec 18 11:24:13 localhost kernel: [11090.782592] iwlwifi 0000:0c:00.0: Not sending command - RF KILL
Dec 18 11:24:13 localhost kernel: [11090.795404] iwlwifi 0000:0c:00.0: RF_KILL bit toggled to disable radio.
Dec 18 11:24:13 localhost wpa_supplicant[1107]: rfkill: WLAN hard blocked
Dec 18 11:24:13 localhost NetworkManager[1109]: <warn>  [1545150253.0043] sup-iface[0x557818a4da10,wlp12s0]: connection disconnected (reason -3)
Dec 18 11:24:13 localhost wpa_supplicant[1107]: wlp12s0: CTRL-EVENT-DISCONNECTED bssid=24:b6:57:f9:c6:9e reason=3 locally_generated=1
Dec 18 11:24:13 localhost systemd[1]: Starting Load/Save RF Kill Switch Status...
Dec 18 11:24:13 localhost avahi-daemon[1078]: Interface wlp12s0.IPv6 no longer relevant for mDNS.
Dec 18 11:24:13 localhost NetworkManager[1109]: <info>  [1545150253.0065] manager: rfkill: WiFi now disabled by radio killswitch
Dec 18 11:24:13 localhost avahi-daemon[1078]: Leaving mDNS multicast group on interface wlp12s0.IPv6 with address fe80::44c3:6970:aa31:f9a2.
Dec 18 11:24:13 localhost NetworkManager[1109]: <info>  [1545150253.0069] device (wlp12s0): state change: activated -> unavailable (reason 'none', sys-iface-state: 'managed')
Dec 18 11:24:13 localhost avahi-daemon[1078]: Interface wlp12s0.IPv4 no longer relevant for mDNS.
Dec 18 11:24:13 localhost NetworkManager[1109]: <info>  [1545150253.0144] dhcp4 (wlp12s0): canceled DHCP transaction, DHCP client pid 10463
Dec 18 11:24:13 localhost avahi-daemon[1078]: Leaving mDNS multicast group on interface wlp12s0.IPv4 with address 10.100.41.244.
Dec 18 11:24:13 localhost NetworkManager[1109]: <info>  [1545150253.0144] dhcp4 (wlp12s0): state changed bound -> done
Dec 18 11:24:13 localhost avahi-daemon[1078]: Withdrawing address record for fe80::44c3:6970:aa31:f9a2 on wlp12s0.
Dec 18 11:24:13 localhost systemd[1]: Started Load/Save RF Kill Switch Status.
Dec 18 11:24:13 localhost avahi-daemon[1078]: Withdrawing address record for 10.100.41.244 on wlp12s0.