1. A computer-implemented method for data loss prevention for text fields, at least a portion of the method being performed by a computing device comprising at least one processor, the method comprising: identifying, by the computing device, a form submission sent from a client system, the form submission comprising a textual field; storing, by the computing device, at least one characteristic of a value of the textual field within the form submission in connection with an identifier of a form used to generate the form submission; determining, by the computing device and based at least in part on the characteristic of the value of the textual field, that the textual field comprises user-generated content; storing, by the computing device an additional characteristic of an additional value of an additional textual field within the form submission in connection with the identifier of the form; determining, by the computing device and based at least in part on the additional characteristic, that the additional textual field does not comprise user-generated content; intercepting, by the computing device and a subsequent form submission derived from the form; subjecting, by the computing device, the textual field within the subsequent form submission to a data-loss-prevention analysis based at least in part on determining that the textual field comprises user-generated content; omitting, by the computing device, the additional textual field from the data-loss-prevention analysis based at least in part on determining that the additional textual field does not comprise user-generated content; performing, by the computing device, a data-loss-prevention action based on the data-loss-prevention analysis.