why does "text message forwarding" option suddenly stop appearing in iPhone settings?

If you google any version of the title of this post, you will find dozens of articles saying how to fix the problem. Turning iMessage off/on and signing back in is the easiest and most reliable fix. There is no shortage of uniformed suggestions for the fix, but there are *no* articles explaining why the problem occurs in the first place on a device on which the option has been active for months or years.

This problem has appeared on multiple iPhones, multiple times, under multiple IOS versions over at least the past 3-4 years. All with Verizon, all with same Apple IDs. My guess is that this is a server problem somewhere in the Apple infrastructure, but no one explains why or what the cause is. Least of all, no Apple support article gives any hint that this might be a bug or server problem. What I don't understand is why there is no knowledgable explanation as to causes to that we users can be alert to conditions that might lead to this broken behavior.