robots.txt isn't provided, that's an individual choice for each site what to allow or block from being crawled. The one that XenForo uses is here https://xenforo.com/robots.txt.
It's probably similar for XF 1.5 except for the whats-new entry which I think used to be find-new. Other than that, search this forum for other threads on robots.txt or look for XenForo 1.5 sites to see what their robots.txt looks like.