This depends on when the ancients. Flat and even rhymes belong to phonology. After a general understanding of the development of phonology, we can know when a flat rhyme is formed, thus standardizing the creation of irregular poems and then becoming the standard to measure the fluency of poems.
The origin of phonology is very early, but it didn't start from the Book of Songs. At that time, most of the creations were arbitrary, just need to sing and finally rhyme. This poem has no requirement for words, but they are fluent and expressive. This is the standard of classical poetry.
During the period of Cao Wei, Cao Zhi, a gifted scholar, pioneered figures of speech, with beautiful and delicate words, lofty spirit of poetry creation, gorgeous words, elegant feelings and outstanding features. He has made great achievements in prose and poetry, and he is also called "immortal genius" with Li Bai and Su Shi.
In fact, Cao Zhi is also the originator of phonology. During the Eastern Han Dynasty, the White Horse Temple was built and Buddhism flowed into the Central Plains. But Buddhism sings Buddhist scriptures for everyone, using Indian accents and tunes. However, after the scriptures are translated into Chinese, they will be out of tune. It's as if we are translating European and American pop songs into Chinese now. If you don't rewrite the words, just sing like that, it must be a strange tune.
When Cao Cao came to power, he had doubts about this talented uncle. Cao Zhi had to avoid suspicion and devote himself to studying Buddhist scriptures. In order to facilitate singing, he translated Buddhist scriptures with his own literary talent to match the original sound of India. In this process, he discovered some rules of Chinese pronunciation and created Sanskrit, so Sanskrit is alive. This is the beginning of China's phonology.
Because Cao Zhi's literary talent is innate, his literary words embellish the genre and phonological followers, and poets begin to pay attention to the beauty of words and the tone of reading aloud, and poetry creation begins to move towards an ingenious road. Coupled with the Southern and Northern Dynasties after Wei and Jin Dynasties, Buddhism became popular in the Southern Dynasties and even became the state religion. The study of phonology has mushroomed, and many rhyming books and researchers have appeared.
For example, Zhouyi's four-tone rhyme, Shen Yue's four-tone spectrum and Wang Bin's four-tone theory; Then there is Four Rhymes Forest, Liu's Four Rhymes Forest and Xia's Four Rhymes Forest. These researchers have their own emphases, but they have the same cognition that Chinese characters are divided into four tones.
Among them, Shen Yue is a great poet. He linked phonology with poetry writing, and used the laws of phonology to create harmony and loudness in poetry reading and music. Shen Yue's "Four Tones Spectrum" puts forward that four tones are "going up". When these four-tone words are used at intervals in poetry, they form a cadence and catchy feeling.
Good news travels fast. Since Shen Yue, poets Yu Xin, Bao Zhao, Xie Lingyun and Xie Tiao have been supporters of four-character poems. The rise of the folk, and the court of the Southern Dynasty paid attention to pleasure, supported this method of beautifying the musicality of poetry, and became the mainstream with four tones.
Poetry developed into the Tang Dynasty. Through the early Tang literati's criticism of the Qi Liang style in the Six Dynasties, the poetic style has been reformed and affirmed, and the themes are rich and diverse. The court poets created metrical patterns in four tones, antithesis and bonding. The formation of modern poetry (metrical style).
Xu, the prime minister at that time, found that these four kinds of rhymes were more complicated than metrical poems, so he presided over the compilation of a verse at the behest of Wu Zetian. This book directly simplifies the four-tone rhyme against metrical poetry into flat tones, and divides the "flat" and "rising tone into tone" in the four tones into flat tones. This is the origin of today's Ping, and it is also the basis of Liu Yuan's Ping Yun Shui in the late Southern Song Dynasty.
Therefore, leveling itself is aimed at metrical poetry. Before the appearance of metrical poems, everyone kept the four-tone rhyme and would not consider the nonexistent concept of "leveling". Before the birth of phonology, poets could not abide by these things when writing poems. However, these laws are summarized from poems, so as long as they are cadenced and catchy, they are unconsciously observing the tone rhythm of Chinese characters.
Flat and even meter is actually a tool for writing poems summed up by previous poets. If you want to learn metrical poetry, you can't get around it.