Currently there hasn't been any official reports as to why this occurred. None of the official social media accounts for the emulator have given any response, nor has any of the developers talked about why the repository is down, but we'll be sure to keep you updated whenever any new developments towards this situation are known.
UPDATE #1: Given the nature of the page being down with a 404 message, it's possible that this could be related to the removal of the Ryujinx account, and not a DMCA. If the repository suffered from a DMCA claim, then the DMCA notice would appear when entering the repository. However, that is not the case, and the DMCA listings from GitHub doesn't have any recent entries for Ryujinx.
UPDATE #2: The Ryujinx Discord server has paused invites for the moment, and additionally, one of the Discord mods posted the following message:
"To clarify, it's not a DMCA, it's not an issue with GitHub. Please wait for further information and refrain from spreading misinfo thanks."
Ryujinx GitHub Repository