Now we’re getting somewhere, at least I think we are. Your statement “Truth requires that the phenomena remains unchanged if there were become no observers” appears to argue for a kind of objective or absolute truth — something that would be true whether you or I perceived it as such or not, or even perceived it at all - though I fail to see the value of a truth that exists independent of any observers capable of appreciating its existence and relevance. There are things that at least largely meet this definition but they tend to take the form of small and relatively simple objects like molecules with little to no relevance in social science, policy, or moral contexts. Water, for example, would consist of an oxygen atom and two hydrogen atoms anywhere we or anyone else found the thing we call water existing in the universe. Hypothetical aliens would undoubtedly have another name for water and possibly even have a very different experience of it but to the extent they could observe the basic structure of water, namely the molecule itself, it would fundamentally look no different even if they perceive oxygen and hydrogen atoms differently than we do; in that case, water would simply consist of an oxygen atom and two hydrogen atoms as they perceive them. If it didn’t, it wouldn’t be water. So, I agree with you if your point is that this sort of objective unchanging truth is non-existent, at least experientially, when we scale the universe up to complex organisms, especially to ones capable of having subjective experiences and interpersonal relationships.
I’ve never argued the kind of rigid objectivity you describe has been or even can be experienced in the context of complex systems, especially social ones. The presence of observers is, in any context in which truth is a relevant concern, a part of the true overall picture. To argue real truth can only exist independently of observers is a non-sequitur given it assumes the truth can never be known since knowing it would require it to be observed in some form. Using this definition of truth the argument itself can’t be true (or at least we can’t know it to be) given we have made the observation that it is true.
But I reject this whole notion that truth can only take one very narrowly prescribed form in the first place. A Historian, for example, is concerned with accuracy, which is a kind of truth but not one that obviously or consistently fits the rigid definition you offer. Was Lincoln shot in Ford’s Theater, for example, yes or no? I have no direct personal subjective experience that’s relevant to finding the answer to that question nor does the historian, though indirect experience abounds. That said, there was a time when it was true that the answer to that question was no and then a time when it was yes, so to say the answer hasn’t changed would clearly be incorrect. We could complicate the question further by referring to other people that go by the name “Lincoln” and add that “it depends upon which Lincoln we’re talking about.” In that case, no may be the correct answer in one instance but yes the right answer in another. Naturally, if we were able to ask aliens living on the other side of the universe that question they would wonder who Lincoln is and ask us questions like “where’s Ford’s Theater?”, etc. But, of course, the fact that our hypothetical aliens would lack all relevant personal and historical context does not by itself render the question unanswerable. Nor does the fact that here on earth different people have different subjective experiences of Lincoln, both while he was alive and in the present. Truth, in other words, is anything but free from or independent of subjective experience and is even a bit malleable in most cases, though not infinitely so. If that’s your original point then we at least largely agree but if you’re embracing the objective and unchangeable definition of it exclusively we don’t. If you want my views on this subject I suggest you read my article Objectivity vs. Subjectivity: An Incongruity That Isn’t Really.