reviewing these comments, it appears to me that there is a consensus that 'exterior stairs should require handrails'. i also assume from these comments, that nobody so far can cite or knows where else to look for a relevant Code, Standard, or a committee paper which would require/recommend this. i also agree that codes don't stop us from doing something better. But, i would also say that codes are for setting a minimum and it seems weird to me that such a common and simple item appears to have evaded the code process. the part of my brain which is responsible for rampant speculation has been considering the possibility that this issue was not codified because it could affect the national, state, and local parks in a bad way.
i've already dealt with my individual situation (fyi, i advised for handrails), but i believe the value of these forum Q&A's are about 1% for the benefit of the OP and 99% for the benefit of the 100k+ of designers, most of which non-members, who end up here by a google search of keywords. although my issue is resolved, the issue appears to be open. . Future responses on the topic are absolutely welcome.