Disclaimer: I have a mostly Mahayana background, so correct me on any mistakes.
In the 5th Century, Buddhaghosa wrote the Visuddhimagga, which was the manual on Theravada meditation techniques and exegesis on its theology. It was touted as a step-by-step path to Enlightenment.
But Buddhaghosa in a postscript stated that he did not believe Theravada practices could lead to Nirvana. He hoped that by writing the Visuddhimagga, he would earn enough Merit to be reborn in a Heavenly Realm to await Maitreya’s arrival, so that he could hear Maitreya preach, and thus gain enlightenment. This postscript betrayed Buddhaghosa’s soteriological beliefs.
In addition, by the 10th Century, Theravada Buddhism no longer practice vipassana meditation at all, because they believed that the Buddha-Dharma had degenerated and that enlightenment was no longer possible until the arrival of Maitreya.
In essence, it seemed that for a least eight centuries, Theravada was almost Messianic.
In the 18th century, Vipassana Meditation was re-invented in Myanmar by Medawi, a Burmese Monk who is credited as the first author of the modern vipassana movement.
Reform & Decline of Tantric Theravada
In the intervening period before modernity, most Theravada meditation had been based on the Yogāvacara tradition (also known as borān kammaṭṭhāna, Southern Esoteric, or Tantric Theravada), but it declined, and was suppressed by the Reform movement in the 19th century which took Buddhaghosa’s philosophy as the orthodox interpretation of Theravada, as well as by French Colonial suppression and then by the Khmer Rouge. The Vipassana Meditation movement displaced Tantric Theravada.
The reforms were partly a reaction towards Western Colonialism, but also relied on the proliferation of the Pali Canon by Western interest to realize. The Theosophical Society played a big role in reviving the faith locally, especially amongst the laity. Theravada, however it existed in the past, was reshaped according to modern interpretations of the Pali Canon by modern eyes and modern minds.
There are also the Weizza (vijjādhara), or Buddhist Wizards in Burma, which may be related to Southern Esoteric Buddhism. Weizza practices alchemy and magic in order to attain timeless immortality; their purpose is to live long enough for Maitreya to arrive. This fits into the pre-reform beliefs about being unable to attain liberation without Maitreya.