No, its not a "required"... It means someone may have reasons not to use something, and so spec implementors need to allow for circumstances where it is not present.
replyThose reasons can be anything. Legal, practical, technological, ideaological. You don't know. All you know is not using it is explicitly permitted.
> You don't know. All you know is not using it is explicitly permitted.
replyIn theory, if they are truly following the specification, you know they thought hard about all the consequences.
I think the pushback in the comments comes from the commonsense feeling that this... didn't happen here.
I don't even know how you got to "used twice" tbh. Both your own comment AND the post you quoted from only have a single "must".
replyThe only thing that text demands is understanding and carefully weighing the implications. If, having done that, you conclude that you don't want to then there is absolutely nothing in the spec stopping you. Maybe the spec would have been better off putting more stuff in SHALL and less in SHOULD, but as written that is definitely not the case.