I'm dubious about using "protocol" in an English translation: it doesn't seem to exactly fit any of the definitions given in Wiktionary, and Oxford translates
protocolo in the context of law as
registry. The English Wikipedia page on
notaries does use the term, but reading the page as a whole suggests that a better translation for
fuera de protocolo is
in private form.
For the entire phrase, I suggest the translation
Notarial instrument in private form of witness statement.