Khamba Thoibi is "a legendary Meitei language epic poem" and "is regarded as the national epic of the Manipuris". The Wikipedia article about Meitei literature makes no mention of a national epic but says that "Khamba Thoibi is regarded as the greatest epic poetry in Meitei literature." However, the same article also contains a section on suppression of Meitei literature:
After the adoption of Hinduism as state religion under Gharib Nawaz (1717), it appears that the Puyas were "burnt completely" at Kangla Uttra under royal orders, in either 1729 or in 1732.
Based on the Wikipedia article Puya (Meitei texts), it is not clear whether Khamba Thoibi was considered part of this category of texts (nothing indicates that it was). It does not seem that Khamba Thoibi was suppressed as a text. Nevertheless, it is worth asking when Khamba Thoibi started to be seen as a national epic. For example, did this happen at the time of the upsurge of Meitei nationalism in the 19th century as part an effort to create an invented tradition? Or should we look at a different time period in the history of the Manipuris?