What adjective can I use for things that in theory can be assumed but in real world never happen? For example, consider "different types of societies". One of the types that we mean is a society in which the thoughts and beliefs of all the members are exactly the same. Another type is a society in which each members has thoughts and beliefs that are totally independent form others. Obviously, these types can be assumed but never happen in the real world. Is "different theoretically possible types of societies" appropriate?
You're not going to find a word that means "could exist but will never exist." Something (1) does exist, (2) does not exist, (3) is capable of existing, or (4) cannot exist.
However, there are some words and phrases that might still apply:
unlikely to be true or to occur; also : unlikely but real or true
of, relating to, or consisting of concepts
The implication is that it does not exist in practice. (However, it may not be appropriate because concepts often turn into reality.)
Despite it's literal meaning based on practical ("of, relating to, or manifested in practice or action : not theoretical") the phrase practically impossible actually means essentially or effectively impossible, not absolutely impossible.
"Utopian" is an adjective that describes societies, and aspects thereof, that are someone's theoretical ideal, but anyone with sense recognizes as being impossible.
The word is derived from the title of Sir Thomas More's 1516 book Utopia about a fictional version of England. He chose the name "Utopia" to literally mean "no place" in Latin.
"Dystopian" is an adjective that describes societies, and aspects thereof, that someone thinks are very bad. It is often used to describe science fiction set in fictional worlds that are worse than today's world.
The word is derived from the prefix "dys" (as in "dysfunctional") and the word "utopia".
"Platonic ideal" is a noun phrase or adjective phrase that describes the theoretically perfect version of something. In practice, everything in reality is at least slightly different from its platonic ideal.
The phrase is derived from Plato's philosophical discussions of the difference between theory and reality.