MarsSeed [1] filed a deletion request for waifu2x-converter-cpp-cuda-
git [2]:

Development discontinued since 2021, repo archived in 2022. [a]

Therefore there's no need to keep this VCS build. AUR still has a
maintained package for the latest stable release. [b]

[a]:
https://github.com/DeadSix27/waifu2x-converter-cpp
[b]:
https://aur.archlinux.org/packages/waifu2x-converter-cpp

[1] https://aur.archlinux.org/account/MarsSeed/
[2] https://aur.archlinux.org/pkgbase/waifu2x-converter-cpp-cuda-git/