Dug a wee bit, pacman -Qi ibus-ui-emojier-plasma comes up with nothing, as does pacman -Ss ibus-ui-emojier-plasma, meaning it’s not a package of its own. pacman -Qo ibus-ui-emojier-plasma tells me it’s in fact owned by plasma-desktop, and that obviously doesn’t have noto-fonts-emoji linked to it in any way.
However, both plasma-integration and qt5-webengine require noto-fonts, of which noto-fonts-emoji is an optional dependency.
All of the above packages are maintained by the same person, so might want to leave them something on the Arch forum and hope they read it (might not want to email them, that’s bad manners 
).