bibtex-user-optional-fields is a variable defined in `bibtex.el'.
Its value is
(("annote" "Personal annotation (ignored)"))

  • This variable may be risky if used as a file-local variable.

List of optional fields the user wants to have always present.
Entries should be of the same form as the OPTIONAL list
in `bibtex-BibTeX-entry-alist' (which see).

You can customize this variable.