I’m not a lawyer. Just a mechanic. But to the best of my knowledge and understanding, TREAD has absolutlely nothing to do with tire installation or repair procedures. No tire manufacturer approves of the old “rope plug” method of repair, but that has to do with the fact that it doesn’t require breaking the tire down and inspecting the inside of the carcass for damage.
Having said that, I don’t know how many tires I’ve plugged in 25 years, and it’s perfectly safe when done using some common sense and for the appropriate type of puncture.
Plug away my friend.