At least one legal opinion suggests there are circumstances under which the robots.txt file of a website qualifies as a technological measure to control access to copyrighted material (at least if access is pursued via the Wayback Machine):
The protocol by itself is not analogous to digital password protection or encryption. However, in this case, when all systems involved in processing requests via the Wayback Machine are operating properly, the placement of a correct robots.txt file on Healthcare Advocates’ current website does work to block users from accessing archived screenshots of its website. (Pl’s Mot. Partial Summ. J. Ex. F, Expert Report of Edward Felton at 10). The only way to gain access would be for Healthcare Advocates to remove the robots.txt file from its website, and only the website owner can remove the robots.txt file. Thus, in this situation, the robots.txt file qualifies as a technological measure effectively controlling access to the archived copyrighted images of Healthcare Advocates. This finding should not be interpreted as a finding that a robots.txt file universally qualifies as a technological measure that controls access to copyrighted works under the DMCA.
Note the final sentence which is interesting – I assume Judge Kelly wants to significantly qualify the opinion. Makes sense to me.
A interesting aside is the reference to a case where the finding was that "the unauthorized use of a password did not constitute circumvention under the DMCA."