What ever happened to the Chinese Indians who were deported during the Indo-Sino war?

I can address at least some of the population, formerly residing in region where I spend my time. I work in Upper Assam and Arunachal, mostly with groups from the Scheduled Tribes, as well as in Upper Burma in places with large Chinese-speaking populations. If you'll forgive me, I'll go a little into the backstory which you may already know but which other people reading this might not.

Unfortunately I can't really tell you much about what has happened after leaving India other than people trying to get on with their lives while seeking acknowledgement of the atrocities that had happened. I know the follow up is more of your question, but some of this may still be of value to you. Not all were deported.

There are a few different threads going on with Chinese in that region. One is with Chinese who arrived as part of the diaspora, under the same pressures that motivated the diaspora in places like California and Vancouver, and one is with "Chinese" coming over from Yunnan and what is now Sagaing. I say "Chinese" in quotes here because, for people from the former NEFA who travel anywhere outside the former NEFA in India, it's quite common for Central Indians (as they are known in the Northeast) to refer to any of the Scheduled Tribes from that area as "Chinese".^1

Many Chinese people first came to Upper Assam and the Northeast Frontier as labourers. Their primary occupation was as workers either in the tea fields, which had only been in production for a short time at that stage (late 19th, early 20th century) or in the other industries common to the area (oil, brick production, coal). This was encouraged by the East India Company, which had an easy time finding workers given the various wars and famines happening in South China at the time, about which I've written on this subreddit before relating to diasporas in the western hemisphere.

South China at the time had undergone two wars with Britain, a civil war between the Taipings and the Qing, poor management by the central government, and a high number of famines, not to mention various other rebellions, and wars fought between different ethnic groups based on a longstanding but unbalanced economic model of land ownership. In general, things were bad there and a lot of people were trying to temporarily get out. Working on a tea field or in a brick yard for a few paise a day was still more money than you could make back home, and it's the reason that period saw so many people leaving South China where things were the worst. Keep in mind many were really migrant workers with no intention to stay long term, but of course some did stay. These groups, those coming over from places like the Pearl River Delta, were focused in Kolkata, and then from there would have gone north into Assam to work, or remained in Kolkata handling the tea trade there. In Assam, just east of Tinsukia there's a town called Makum, not to be confused with Makum that was renamed Margherita. In Makum a small Chinatown developed, and today the area where it stood is still known as, "Chinese strip".^2 By most accounts, prior to 1962, things were fairly amicable, and there was a good deal of intermarriage. I'll get to that more in a minute.

So for many Chinese in the Northeast, the opportunity to work and make a living is what brought them. The second thread of migration to the area came from across the border. From parts of what is now Sagaing Region in Upper Burma and the Arunachal border closer to China, a number of people migrated either seeking better farming or trade opportunities. For them, when the war hit, things were a little easier as they could (and did in many cases) go back over the border. I've met people in towns like Hkamti in Sagaing who had migrated from India at that time, and even today you find people from across the border temporarily making a home in towns in Arunachal for trade, without long-term plans at staying there (e.g. not learning Assamese, using Burmese or Tai unless a smattering of Hindi or English is required).

Finally, some Chinese in the region came later, as part of the KMT army of the Republic of China during World War II. Some settled, for example a large population in Wa State in Myanmar and smaller numbers in India.^3 Those that had come as soldiers and stayed would have merged with existing communities, and probably only exist now in family histories, though there are at least accounts of Chinese on the Burma side relocating to Taiwan.^4

When war broke out in 1962, China made a push for the land that they felt was rightfully theirs, under the argument that Tibet was theirs and part of Arunachal Pradesh was Tibet.^5 Animosity that would have already existed despite an otherwise stable society in the area meant Chinese were no longer welcome or treated all that well. Chinese Assamese were stripped of whatever citizens rights they might have had and often detained. There's been an unnerving trend in the North East of questioning rights of place of minorities, even as recently as a 2016 issue on who gets to count as Assamese and who must be deported. This has not been unique to the Northeast, as the same has been happening throughout the region, but in the many decades since partition it's been a frequent reoccurrence. Those who didn't flee or who weren't lucky enough to go unnoticed were detained. One of the most significant internment camps was in Deoli, Rajasthan.^6 While not the only ones detained there in Modern India's brief history, the detention was significant, with many people being there for years.

After the war, when those in the detention centres were released, they were still basically unwelcome, and very few returned home for long if at all. It was at this point that Chinese Indians began leaving, many to Canada, but for a lot of people it was a slow process. The government had confiscated most peoples' businesses, and since many of the people who might have otherwise passed for Assamese has been betrayed by non-Chinese in their towns, there wasn't much reason to go back. Over the next couple decades more and more Chinese left India.

For many, what came next was a continuance of life, but starting over in a new country. Right to return was also difficult, and today Chinese travelling in places like Arunachal Pradesh are often met with suspicion. There are still significant tensions over the border, with the Indian military regularly running jets along the border in a show of force, and the Assam Rifles presence being felt strongly in the region.

But one point I want to make is that I want to get back to that intermarriage bit. The people in Assam who were detained were generally the ones who were most obviously Chinese, wither by having a Chinese name or by not having assimilated to the degree that many had. However many Chinese people married with Assamese, Tai, Singpho or other groups, and today there are still a number of people who you may find after getting to know them are part Chinese, although this is often less publicised. More something that gets said to you quietly off the cuff about someone in their absence. No one holds it against people, and ethnic identity is a much more fluid concept in the Northeast than in places like the US, with people shifting from generation to generation in a long series of mixed marriages. But at least for some of the Chinese in the Northeast, they didn't go anywhere, they just became less identifiably Chinese after a generation or two.

For more about what some of these former internees are doing, I recommend checking out the Association of India Deoli Camp Internees 1962, who have a fairly strong web presence.

I know much of this may be things you've already read OP, but I hope it's at least informative for someone out there reading it.

Footnotes:

This is the case for many people, be they Adi, Nocte, Wancho, Monpa, Bodo etc. Adivasis and Biharis do not receive this treatment, of course. When I say "Chinese" I mean people from China speaking Sinitic, but who are potentially members of minority groups from within China's borders who, to locals, may not be known to belong to another group identity such as Lisu, but I do not mean the Scheduled Tribes.
চীনা প্যাটি, Cheenapatti or just China Patty. As with all Assamese romanisations, expect variations.
In Jairampur near the Burmese border a Chinese WWII cemetery was rediscovered in 1997. It was briefly uncovered and has gone back into a state of disarray, but when I was there a few years back they had started making efforts at running it as a proper museum, though given how hard it is for Chinese to get that far into Arunachal, I guess good luck.
Frontier Myanmar, a great magazine based out of Yangon, did a great piece on one such guy which I recommend to anyone interested. Pretty sure you can still find it on their website.
If you are outside India, Google Maps shows this as a dotted line running along the Bramaputra by Pasighat and then around and north of towns like Doom-Dooma and Bor Phake in the east, not including Changlang district or any points south. If you are in India, a recent law making "misrepresentation of borders" a criminal offence means you'll only see India's claimed border in that region, which in fairness is also what India actually administers unlike China's claim.
The Wire has a great but short piece on the subject including some photos which I recommend checking out if you haven't read it yet.