"digital system MUST represent any data with binary signals. This is mandatory requirement for digital system"
So you keep saying, despite centuries of digital systems using non-binary components and representations, that predate binary systems.
You may have a limited universe of discourse, but there are many examples in the larger picture, brought to your attention by multiple posters, after which you put your fingers in your ears and recite your mantra.
George Boole introduced his system in 1854, following work by Leibnitz.
In the 1930s, Shannon applied binary Boolean algebra to switching circuits, after which "Boolean" came to mean binary.
Boolean algebra is used for symbolic logic that recognizes only "true" or "false" as valid outcomes; other forms of logic can have further answers in their set of valid outcomes.
Binary arithmetic differs from Boolean algebra, but also uses binary variables.
Modern mathematics recognizes a multiplicity of "Boolean algebras", but that is higher mathematics.
In this forum, there are many opinions legitimately held by individuals that disagree with each other, such as personal preferences for Windows, Linux, Mac, or other operating system.
Different people find different choices better suited to their experience, application, or familiarity.
One of my strongly held opinions is in favor of RPN over "algebraic" notation on pocket calculators.
However, the standard definition of digital (antonym of analog) is that it uses discrete states and representation, instead of continuous.
If someone claims that "there must have been a year zero" in discussing the civil calendar in use in Western countries, that opinion of theirs is wrong, since the civil calendar is otherwise defined to go from 1 BC to AD 1 without a year in between, and a different definition would change the historical date of Caesar's assassination.
(Astronomers do use a different calendar definition, where the year before AD 1 is zero, and years before that are shifted by one from the legal civil calendar, but we don't rewrite the history books.)