It finally landed a few days ago for the Proximus customers
Sorry if this is off topic, but does that mean you cannot update your phone without a (working?) SIM?
Once a SIM was inserted the system will always use those network info, according to my understanding, thus just deactivating the SIM does not work when the network blocks the update somehow, you would need another network SIM. My FP4 e.g. has no active SIM and I still get updates to my understanding based on the last network that its was connected to. If you never install any SIM you still get updates over Wifi… please dont ask me for more details, I dont have them 
Not quite as bad as I feared then 
Thanks for the explanation!
There are community devices with Wi-Fi only receiving updates, I reckon on a regular basis. So it is definitely working.
There is one in Aachen, and it gets updated pretty smoothly, as @urs_lesse can confirm:)