I remember back when I first caught the travel bug back in the late 1990's (yeah, i'm probably older than you) and one of the most popular places to travel as a novice was South East Asia. The free visas, the ease of travel between countries and regions in that country, the wide array of culture and food and of course the cheap prices were a wonderful arrangement of reasons to head there.
While backpacking to this par of the world, as well as backpacking in general, has waned in the past 8 years or so, these days you can't even get into most of the countries at all and it might be a while before that changes as well. I don't know how these decisions are made but for all but Thailand, it isn't even possible to travel to any of the countries unless you are a resident or have some other special exception (being rich probably helps.)
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Red = no
Yellow indicates that you can get into the country if you are double vaccinated or in some specific cases, if you are subjected to multiple PCR or antigen tests and do some sort of state-organized quarantine at your own expense. Bangladesh and Nepal have similar systems in place but that isn't considered SE Asia so I didn't include it.
I know a few people that are currently in all of these countries and even a guy who recently landed in Thailand for the "sandbox tourism program" that they are doing in several places in that country. The people who live in Cambodia and Vietnam have said that they are trying to stay as long as they can because they fear (and are probably correct) that they will not be capable of getting back in once they depart.
The friend of mine who has gone to the Thailand sandbox program is happy he did it, but is quickly becoming bored because although he is experiencing the weather than he prefers and it is cheaper than his native Canada by a long shot, you aren't allowed to do a lot of things. Basically, if you leave the grounds of your resort you are subjected to an almost military type rule where you are constantly being watched and the mask mandates are quite seriously enforced.
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While some restaurants are open, they are strangely not permitted to sell any alcohol. He has said that his resort will still serve booze to you, but they have to be very careful about it and can not serve a beer in a beer bottle, instead they have to serve it in a tea cup or some other glass that can't be seen through in the event that the police come by. The police haven't come by but still, it is a strange policy isn't it? I guess they are worried that people will get too drunk and then the chance of spreading covid increases somehow?
The path of many backpackers back in the day incorporated going to many if not all of the South East Asian countries but the most popular ones were Vietnam, Cambodia, Laos, and Thailand. 3 of those countries will not allow tourism of any sort right now so if you were hoping to get some sort of backpacking experience in what was my original backpacking locale and that lead to me living a nomadic life... forget it. It isn't going to happen.
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This was from a while ago, but what an absolutely ridiculous way to eat
In fact, there are very few countries in the world where you can roam from one country to the next like we all used to do back in the day. In many countries, land crossings for tourism is prohibited even though the country is technically open for tourists. I have no idea why this is but it does seem to be the case.
I would say that heading over to Asia for travel right now would be a very bad idea because all of the freedom and exploration that made those places so much fun in the past are not available and the cheaper accommodation isn't even open as the only country that is even open for tourism has such hefty entry restrictions that the incoming tourists tend to be rather wealthy anyway.
For now, write this part of the world off as a "no go" until they can figure out what the hell they are going to do. Instead, turn your focus towards Central America or Eastern Europe and also kind of forget about country-jumping. Find one that you are interested in and fully explore that instead. Because if it's immense size and wide range of sites, I would recommend Mexico. They are basically allowing anyone with a pulse in regardless of their Covid status.