You seem confused about how the AUR works. There is no "client" like you're talking about that can show the user anything.
There are AUR helpers, but these are completely unaffiliated with arch and the people running the AUR. The canonical, recommended way of installing arch packages is cloning a git repo, reading through the sources and then building it with makepkg. There is no client there that could show the user anything.
how comes gitlab shows custom messages to my plain old git client then?
for example when you rename gitlab repository, or push to new branch, gitlab injects custom text that you can see. Eg. with new URL or where you can create merge request on web, etc...
I assume you're talking about the "remote: " messages? I've only ever seen those on push operations, not sure if they're even available for clone.
Maybe they'd be an option, but then the whole "making sure they've read the message before proceeding" part goes out the window.