While it is often said that bhikkhuni ordination is illegal or banned in Thailand, this claim rests on a very slender thread. There have been no formal pronouncements on the matter by the current Mahatherasamakhom, the governing body of Thai Buddhism, whose authority stems from the Sangha Act of 1962. Those seeking an ‘official’ position must fall back on a ruling issued in 1928.
The legal status of the ruling is obscure. No-one, so far as I know, has tried to test this in court. When Voramai Kabilsingh was accused before the Mahatherasamakhom in 1956, her ‘crime’ was allegedly imitating a bhikkhu(!), not the fact that she had ordained as a samaneri. She was excused because her preceptor was a member of the Mahatherasamakhom(!) The same monk, Phra Prommuni, was also the teacher of the current king when he was ordained(!) Phra Prommuni argued that her light-yellow robe was a different color to that of the bhikkhus, so she was excused(!)
As usual, he has gone out of his way to provide reference and context. Read it in full.