Manchu was one of the official languages of the Qing and there were state policies designed to put it on even footing with Chinese. All state documents had to be copied again in Manchu, all bannermen (including Han bannermen) were required to learn Manchu, and Hanlin Academicians were required to be fluent in Manchu as well. But acculturation ultimately led to more and more Manchus speaking Chinese, and already by the early 18th century, emperors were realizing that Chinese was starting to supersede the Manchu language. Some edicts from the Yongzheng period:

"Edict: The Senior Bodyguard and Imperial Bodyguard always can count on sitting idly while on duty. Compared to that, studying Manchu is much preferable. Of late I have heard some of the newly selected guardsmen abandoning the study of Manchu and using Chinese, of all things, in their joking around. This is highly inappropriate." - 1726

"Edict: The study of Manchu by banner soldiers is of the utmost importance. Hereafter let the members of the senior bodyguard, the imperial guard and all those who guard palace gates and keep watch speak only in Manchu. No Chinese should be allowed. As far as training, when the soldiers are assembled and at the parade ground, they should also be made to use only Manchu." - 1734

Mark Elliot noted by the late 1700s, roughly half of the Manchu bannermen in the various garrisons could speak Manchu or at least speak some words. The exception was in Xi'an, which hosted a major garrison that was frequently sent out to expeditions and therefore maintained strict discipline and standards. For garrisons in southern China, the loss of language ability would have been more serious, as there were many edicts from the emperors decrying how Manchus stationed in southern China were becoming more and more "Chinese" and less and less Manchu.

So the decline of the Manchu language is not so much due to active suppression but rather acculturation which led Manchus to favor speaking Chinese. This process had already started during the 18th century.