Perhaps the best way to state this is that plugs are considered to be “temporary repairs” since they are something less than 100% reliable - they have a tendency to leak.
The “Industry Standard” is a combination plug/patch, where the patch seals the inside (for air retention) and bridges the damaged area and the plug seals the damaged area from outside contaminants.
I am not a lawyer, but I have some experience with court cases, and the term “Industry Standard” is used to describe the proper way to do things - and in this case it means that a plug or a patch (by themselves) would not be considered a proper repair in a court of law and could be cited as a cause for a tire failure. Some would say this means there are “illegal”, but that isn’t quite accurate.
Just an FYI: There have been court cases where a plug was the uncontested source of the tire failure - “uncontested” meaning that even the plantiff’s expert agreed.