I am using promotion plugin https://wiki.jenkins-ci.org/display/JENKINS/Promoted+Builds+Plugin
I just realize that the promotions configurations are not being backed up by thinbackup. To support this, it is just to add an extra file located in jobs/[JobName]/promotions/[PromotionName]/config.xml. nextBuildNumber located in the same folder could be useful too.
I am using thinbackup version 1.7.4