Location of "Export Format" files
I have at some point written a custom Export format on Zotero for Windows, so that I can quick-copy into a reduced bibtex format. I now want to edit this export format but can't find where the (presumably .csl?) file is stored on my computer. Where can I find it?
But for your specific case, I recommend you install the BetterBibTeX plugin, which greatly expands Zotero’s BibTeX functionality, including the option to omit fields from BibTeX export.