The wording “must start with” doesn’t sound like a “recommendation”, and I would think that these tools are right to interpret the standard strictly, and that Github is wrong to allow such domain names.

Whatever happened to "be lenient in what you accept, conservative in what you send"?

Github ignores that rule when they emit such domain names. One might make the point that Firefox and others who in turn reject such domain names are not being lenient in what they accept, but since domain names on the edge of acceptability often are a security issue, I think that a strict interpretation of the standard is the safe choice to make here.

It's a terrible idea that just causes all sorts of intro l interop problems. Web development, for instance, is a mess because of all the various attempts to interpret stuff in some way, versus just failing.