Briefly
Basically supplied oxygen ascents are not legitimate. It is like a 6000M climb with supplied oxygen, though not all ascents use the same amount of oxygen. The excuse, was that it was believed that it was impossible to climb Everest without oxygen. I don't really know the history of that, possibly some change related to experience of oxygen on planes. It had been tried quite early without oxygen, as had other mountains of similar size. It was also a time when a lot of the big rock and mountain climbs relied on tech, and tech was sexy, be it rock climbing gear, or oxygen. That changed big time in the 70s.
In 1978 (not 79), Messner and Habeler climbed it without oxygen. But even after that ascent there was some question as to whether Habeler had been injured in the climb by lack of oxygen, or whether they had made it. In 1980, Messner climbed it again, solo, without oxygen, by a new route, alpine style, no camps, no support, just out for a walk. This was also the period, led by the US, when clean climbing on rock came to the fore, and people started looking critically at aided first ascents, and trying to do them free, or without a rope. This was the norm in some backwaters, but it got a boost from the US scene.
The period of maximum oxygen use matched a period of mountaineering pretty much unlike any other. Expeditions were often led by military people, and conducted along military lines. Great national prestige hung on the outcomes. It was not within the normal rules of the sport, which are similar in concept and origin to "fair chase". These were the methods of the day, so nobody so engaged were being cheats. But it is still like the doping in other sports, that in cycling often also involves messing around with oxygen uptake capability.
At the same time there were other controversies, in which climbers did things like take gas compressors, and rock drills to nail their way to the top of rock walls.
The degree to which assistance always factors into climbing can be seen in Honnold's free solo climb of el Capitan, a good candidate for greatest athletic achievement in human history.
Two other things to say about oxygen, it and the gear for it, is heavy, so there always needs to be support, or mainly does. You need Sherpas, and camps, and logistics. But Messner can come along, basically with a day pack and climb from bottom to top in one go. So the "cheat" is twofold, you needed to have summit climbers supported by other climbers, so it is a team sport. Say I decided to run the NY marathon, and I showed up with 4 other running friends, and we won as a team. Than kind of thing. And then add in the O2
The other aspect is that the expeditions were lucrative for the locals, as are now all the fakers who pay 100K to climb Everest (still a big deal with O2). This has a huge economic and cultural, and pollution impact on these mountains, but that is for the locals to worry about and weigh.
In general, climbers still refer to Hillary's ascent as the first ascent. I have only rarely seen climbers say "when it was first climbed in 1978". People really liked Hillary as a person. But the reality is it didn't happen till '78. And i don't see top climbers still climb it with oxygen, unless they might be guiding. With the distance of time, we will start to see this as a period of a little over 20 years when the exception was briefly the rule. And the Wokies may also see it as a period when rather than moutaineering being the "Freedom of the Hills" is was an adjunct to failing imperial dreams.