I don’t believe such a law exists, unless one has been created in the last few years. There was, apparently, a statement by the Mahatherasamakhom, but that was many decades ago., and it is not clear what force, if any, that holds under law. The senate review found that the ban on bhikkhuni ordination is unconstitutional on two grounds: it violates gender equality and religious freedom, both of which were guaranteed under the Thai constitution. Obviously there’s been a few constitutions since then, but as far as I know, the senate committee finding is still the highest legal advice and would prevail until and unless a Supreme Court or similar found differently.
Indeed, although again you’d have to question to what extent such matters are in fact dealt with in law. I mean obviously there are certain official rules that are followed, for example regarding the appointment of upajjhayas, but in many cases the rules are simply adopted by the Sangha on its own.
The Sangha strongly opposed the upajjhaya law when it was implemented, but by now, it seems most monks assume it is the right way to do things. It don’t know why, it is obviously against the Vinaya, and it has hardly produced a functional Sangha.