It was protectorate but not within Indian union. If you see the Indian map before 1975, it did not included Sikkim. Sikkim's foreign policy and security was under Indian hands, but in all other cases, it exercised self rule. It's status was same as Bhutan. Bhutan was also Indian protectorate. But gradually Bhutan shedded it's protectorate status and turn into full fledged UN member state in 1971. Now Bhutan host foreign embassies in it's soil and also has it's embassy in some other countries. In 2007, Bhutan and India revised their original friendship treaty so that Bhutan now no longer bound to seek 'Indian consultation' to formulate it's foreign policy. Now India can not enact the Sikkim saga over UN member state Bhutan without International backlash and non recognition. Sikkim also could have taken similar journey like Bhutan. But unfortunately it ended up into an entirely different fate. If China taken a more pro-active role in Sikkim, perhaps the events trajectory would have been different.