It was not "TRUE" that the earth was flat.....

The earth has always been round. Some humans knew that along time before the 11th century. That most people believed in a flat earth, or bleeding with leeches, or humours, or that there were four elements, didn't make those things true.

Truth never changes.

It was NOT true then...it was a misconception based on poor/no data!
Please do not confuse opinion with Fact, we suffer more than enough of that from the WH nowadays!

As with so many things it depends on your understanding of the word “truth”. To my understanding, truth is what is rather than what we believe. There is no my truth and your truth. There is just truth, and my perception of it and your perception of it. I realize that another legitimate understanding of the word is “information about reality” (rather than reality itself) but even then, inaccurate information about reality is still not true, no matter how popularly believed.

Truth is, in fact, fluid. When I was younger, it was accepted as scientific fact that Pluto was our 9th planet. We may fancy the notion that ‘absolute truths’ exist, with or without humankind’s awareness of them, and this may be the case. But as we have no other means to observe and describe the universe and the phenomena by which we are surrounded, we simply have to concede that our factual knowledge is forever subject to new information, and thus, may very well change. The philosopher of science, Karl Popper, noted that "all knowledge is provisional, temporary, capable of transition at any moment." This would, by necessity, include that which we call 'facts.'

Oh no. The earth was always round. the facts of the physical world are not changed by our beliefs or ignorance.

Truth is demonstrable; can be demonstrated with facts.

If a person believes in for example: the (not testable) supernatural and asserts it as truth
(i.e. Because I want to believe (have faith) in a magic sky daddy I will claim it is the revealed "Truth". Christians often Capitalize the "T" in Truth. The fact that they dishonestly assert their faith (without evidence) claim as truth does not demonstrate their claim has any truth value.

When I hear "fluid truth" I am reminded of a discussion I had with a religious elder (Native American). I pointed out that his culture had what I consider to be a vastly superior vision of reality before the Christian dogma most now embrace. I asked him how he can reconcile this different generations believing different things.

He replied, "Truth is whatever you believe" - - this sounds a lot like your "fluid truth". I find using the "trump era" alternative truth (a belief that is accepted as true) definition deludes the "that which is true or in accordance with fact or reality." definition.

Truth is always there whether or not people know about it.
The Earth since life began on it has always been a sphere.
Gravity was always a fact.
The Earth has always revolved around the sun.
Etc.

Facts are facts and truth.
Just because someone is ignorant of the facts does not mean that there are other "truths".

But kudos for attempting to validate "alternative facts" as a thing.
It's just never going to happen.

I would call that belief. It was believed that the world was flat without any evidence. Actually even then there were a few that believed otherwise. Truth is usually spoken of as something that can be demonstrably proven with some sort of evidence otherwise it is belief. People speak of "my truth" or in your example, belief that is accepted as true, which I would still say is belief. That is why it's always good to ask people to state how they think a word should be defined.

I would change the wording to fluid understanding. Many Greeks in BC estimated the size of a round earth and their books misplaced or lost or hidden. And people like Copernicus were understood at the highest levels of the Church to be correct, but the Church had to prepare for a fundamental change in philosophy. A lot of people don't know, but the Catholic church operates one of the largest telescopes in the world, in South America. It does mostly science research, but at its core the believers want to find the end of the universe and possibly proof of god.

I don't think that facts about the world can be true or false. We assign these values, the world could give a fig.