Edward C. answered 08/22/22
BM,cum laude with post graduate studies, 50+ years of teaching
Your question is a complicated one. The teerm Clkassical refers to a broad range of muisic written between 1700 and roughly 1920. Music during this time evolved from a very simplistic format and harmonic structure to very complex forms and even more complex harmonic structures by the year 1900. What we call tonality in the earliest classical works was a rather uncomplicated system,for the most part. There are always exceptions to everytrhing.Bach's harmonies are good examples of the music of his era. But, even he was somewhat experimental. Look at the 24th fugue of Book 1 of the Well Tempered Clavier. The subject uses all 121 notes of the chromatic scale. This is something that was also put forth at the end of the 19th century by Schonberg in his theoretical 12 tone system of composition.
The composers of the first half of the 20th century were a very diverse group. Debussy and Ravel were dealing with Eastern harmonies, whole tone scales and the like. Theyh also experimented with polytonality. As did other composeres of those years. Jazz was becoming popular and Gershwin was a proponent of its use in his music. Aaron Cop[land also experimented with jazz. As did the French composwer Milhaud. Edgard Varese was an iconoclast.After World War 2 the public rejected anything that was too dissonant. So concerts stopped programming those works. Then composers decided to become minimalists. They also started to return to tonalities likle the older composers. Only one problem, they were just mimicking them.
So, if tyou are intewrested in being a composer find a new voice to express yourself.