Cambridge dictionary lists "shorthand" as an uncountable word.
However, I see a lot of its usage with the indefinite article. For example:
- Another was labeled "Braves," a shorthand for ICE. - The New York Times
- Developers have a shorthand for this style: the "classic graybox". - The New Yorker
- There's a shorthand for this type of guy. - The Guardian - Film
- "Bach and Beethoven" has become a shorthand for Western classical music. - Britannica