There’s a seed of something worth discussing buried in there - historical cycles tend to suggest that society is most tolerant of differences when things are at their zenith and their nadir.
At the nadir, cooperation with everyone for survival becomes paramount, to the point where refusing to overlook differences can cost you big time. During the rebuild, everyone needs to be onboard with a certain vision of what society should become, and lack of conforming is now what can cost big time. In addition, there’s a heavy focus on breeding if there was a population crash in the past, which can cause issues with people who aren’t particularly inclined to do that. During the zenith, there isn’t any inherent cost to being different, though during the previous step it likely created a bunch of now-unnecessary restrictions which hurts a lot of people (but “different” people even more so). So now there’s a gradual removal of most of those restrictions, allowing people to see that differences aren’t necessarily bad. During the decline, people are desperately grabbing resources and flailing about, and are more inclined to sacrifice or steal from the “different” people if they have to do it to someone. Real history is a lot messier than this, it doesn’t always go in nice neat cycles or work out quite that way if it does. It’s more of a general tendency than a pattern, but it’s a thing a lot of people have noticed.
So in a sense, “luxury morality” could be a metaphorical term for a mode of thinking which comes up when times are good for most people, where a high percentage of the population don’t see any particular need to oppress/abuse the “other” for any sort of “greater good”. That said…
The OP seems to be using “luxury” in a different fashion here: implying that it comes with a high cost, which society can only afford when times are good. And possibly with the additional suggestions that things would be even better for most people if the cost were not paid, and that it’s not a good reflection of “real morality”. If so, that’s missing that oppression does come with a cost of its own. What really matters, then, is what you idealize. If you want a world where everyone can be themselves, tolerance is a low cost (for you and people like you) and high rewards (for your idealized society), while oppression is high cost (for you) and low rewards (for society). On the other hand if you prefer one where people conform to a particular narrow set of practices and ideals, tolerance is high cost (for you) and low reward (for that society), and oppression is low cost (for you, but not the people being oppressed) and high reward (for that society).
In that sense, “luxury morality” could easily be either “a moral framework which is easiest to implement in good times” or “a moral framework which comes at an unacceptable cost” and both could be describing the exact same situation. It just depends on who’s saying it, and why.