The spinach test

I have a question about the spinach test – first of all, I will define what exactly this test is trying to prove from what I understand: the test is to check whether a certain claim is objective or subjective and the conclusion from this test regarding morality is an objective thing because we are not laughing when we say "It's lucky I didn't live 300 years ago because then I would have enslaved slaves."
The question is about another case, this case, which actually speaks from my personal perspective and the time in which I live – really, enslaving slaves is something that is objectively immoral, and we project this moral norm onto everything that has ever happened.
But if we go back in time to Sweden, for example, where (if I'm not mistaken, that's what it was, but even if not, for the sake of the discussion) the custom was that a person who reached old age would "relieve" the public from carrying his burden and jump off a high mountain or something, and if he didn't want to, they would "help him" (all for the public's benefit, of course). And if we come to those people now and tell them that this is immoral, they will burst out laughing no less (or even more) than the spinach thing, what's immoral about that?! This has been the custom since forever, and there is a public that needs to survive, and it's impossible for that old man to be their burden!
The same thing can also be said about the future, let's say that one day a new moral norm will be established in which picking plants (or eating animals) is clearly immoral and is even considered murder (after all, a plant is a living thing). If we now apply the spinach test to this period – "How lucky I am not alive 300 years ago because then I would have eaten lettuce and that is cruel" – I don't think there is anyone who has read this and not laughed out loud.
The conclusion that emerges from this discussion is that when you put the spinach to the test, the result is that you get a normative objective value for our time that we apply to every period, but as mentioned, it is actually subjective because it has no definition and is objective only in our eyes.
And regarding morality in general, isn't this something that is a kind of modern invention? Did previous generations think in terms of morality at all and not just in terms of survival? It is clear that in the beginning, people did not think in terms of morality and did not commit to anything except their own survival, and if morality developed over time and reached its perfection only in our time or so, then perhaps it is really impossible to judge previous generations in terms of morality, which is actually objective. There was simply no perception of it or a partial perception of it in those generations. And perhaps it is actually possible to say that morality is objective, and in this the test of the spinach is correct.
But it is still possible to understand that what we define as morality and in our opinion is good and right – previous generations also defined it as morality and the good and right thing. And future generations will also define it this way (as in the case with lettuce). And then really, according to this, our morality is still incomplete and we are not moral, because we eat lettuce, and if we do, we are just like the ancient Swedes (that is, immoral!).
In conclusion: the seemingly spinach test fails miserably here, and morality is not really something objective, if it is anything at all.
I would love to hear your opinion on the matter.