Until you mentioned it, I never realized that this was a user option provided in the old forum software. After some research, I've discovered that this is not a native feature of the xenForo platform. This response is from one of their developers:
That would cause problems with URLs as the page number is part of it and the page number is based on the quantity of posts per page.
I can look to see if there is a plugin/addon that will address this, but in the meantime, the posts per page setting is something that can only be set globally. Sorry for the inconvenience.