How does your site score on this audit?
It may contain remnants from previous versions of your site, and/or imported from other modules that aren't referenced anymore, and/or used to be a part of testing (i.e., they were used temporarily).
How does GTmetrix trigger this audit?
Read this documentation to learn more about code-splitting.
2) Dead code elimination
3) Dead imported code elimination
However, there are plugins that can help you automatically remove unused code from popular libraries after you've imported them. Click here to learn more about these plugins.
Note: This is an expert-level optimization