Thank you all very much for your replies. So that I am in sync with everyone: the phone I am refering to is an iPhone 5 running on iOS 10.3.4, the latest version available for this device. The point of this discussion is to make use of an old device as, Mr. IdrisSeabright puts it, an iPad. I have no need and have no desire to use this device as a cell phone. An Internet connection via wi-fi is plenty useful.
Please correct me if I am wrong:
1) It is not the device, but the OS which is demanding that a sim be inserted. After the device is powered on, it functions to its full capacity other than cellular with a wi-fi connection. As long as the device is not turned off, the "no sim" message will not appear (except when exiting airplane mode).
2) If put in airplane mode, wi-fi is also halted, rendering the device useless other than for offline functions.
3) A valid sim, i.e., one which connects to some carrier's network is required to fulfill the iOS requirements. If an old sim is placed into the phone which is no longer under contract, then a message "invalid sim" is issued.
(Bonus for your DIY folks: it is possible to use a mini-sim in a nano-slot! CAREFULLY trim the mini-sim's edges to nano- size without damaging the gold fingers. Don't forget to cut a tiny triangle off of the sim from the right corner. Handy for getting a 3G or 4G sim.)
Y'all take care