I see you reposted, and I'm glad you did, because I'm wondering about your question. I've been doing some reading and research and so far I haven't come across any suggestion of a ban on Egyptian in the 4th century by the Church. Would you please provide some more context or a source about this question?

While I can't speak to the ban on Egyptian, the answer to your second question is a resounding "yes". In addition to some very significant modern efforts which I can't get into here (20 year rule), there have been some well-documented historical efforts of intentional language death. The one that comes to mind first was the calculated efforts by the Japanese Occupation of Taiwan from the 1890s until the late 1940s.

Specifically, the aboriginal Austronesian languages were prohibited, and a large number of the different aboriginal groups, collectively known as the "plains tribes", lost their languages as a result. There were also pretty significant efforts to limit the Chinese languages of Hakka and Hoklo in Taiwan at that time as well, including limits on use in public and an outright ban on the use of the languages in newspapers. However the speakers of Chinese languages really fared much better than the aboriginal population. Today, the remaining speakers of Austronesian languages in Taiwan are primarily "mountain tribe" (as opposed to plains tribe) speakers, who thanks to geography were less directly effected by the Japanese language policies.

This Wiki page lists the plains tribes. Of that list, Kavalan has a few remaining speakers, but not many and not located where they once were.

can you elaborate on the ban?