Solved
Why am I getting this yellow warning message...

This is the error message I receive when attempting the "user" function not the a!user function:

The user function does not have a domain prefix....

This is the error message I receive when attempting the "user" function not the a!user function:

The user function does not have a domain prefix....
the user function does have a domain - it's fn!
If you write it as fn!user() that message will disappear.
Some context: these are the available domains:

...and it's now been deemed best practice to prefix Appian constructs with their respective domain.
(just realized fn! doesn't appear in the above list, which seems to me an oversight! - I'll raise this with Appian Engineering)
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