For example, can I say, I like your use of the word riverine as metaphor? Or, must I say, I like your use of the word riverine as a metaphor? Grammarly seems to think that the article "a" is required, but I am certain I have heard it used without an article.
Grammarly is too prescriptive. The article is often present but is not obligatory. For example, when attributing the nature of metaphor to something there is no need for an article. To take one of many examples from ngram:
“Clothing As Metaphor” is a book examining the way clothing may represent religion, society, power and other things.
In the same way one might write “accountancy as crime” when examining the ways in which accountancy can be illegally misused.
Cambridge gives a good example in
an expression that describes a person or object by referring to something that is considered to possess similar characteristics
Ghosts may seem to elude serious scholars except as metaphor or atmosphere, but there is a surprising abundance of books about them.
… (the childish or adolescent as identification model and metaphor for a social sensibility that is both point of departure and recipient for the works.
Google ngram shows increasing usage of “as metaphor” towards the end of the last century and provides many examples similar to the above.