Apple - Terminal/iTerm Autocomplete "greyed" functionality?

This is a feature of the fish shell (Friendly Interactive Shell).

fish's scripting syntax is also quite a bit different from bash and zsh's. If you use zsh, you can also try the zsh-autosuggestions plugin on Github.

There is also a plugin for oh-my-zsh: https://github.com/zsh-users/zsh-autosuggestions. It explicitly aims at Fish-like functionality.