Even though Sawtooth Software survey software does not have a built-in suspend function, if the respondent is using a unique password to start the survey, this functionality is already in place. As soon as a respondent completes a page and hits the "next" button, the results are sent to the server and saved in their database record. If on the next page, they decide to stop, pause, or suspend their survey, when they use the initial link to restart the survey (and type in their password unless it was embedded into their link), the survey will resume where they left off. It doesn't remember the answers to the questions on an unsubmitted page, but it does remember the questions on previously submitted pages.
Please see the "passwords" multi-page help section of the help files for more information. https://www.sawtoothsoftware.com/help/lighthouse-studio/manual/index.html?hid_web_passwords_intro.html