In Jenkins, the run directories were named with datetime, only the build.xml was containing the run number. JENKINS-24380 (or PR: https://github.com/jenkinsci/jenkins/pull/1379) made the directory naming same as run number.

It was introduced in 1.597 – released on 2015-01-19 (release notes). Migration guideline was published in wiki

This was a breaking change, and RunIdMigrator handled directory name change, and updating all links etc.

It was also shipped with a tool to unmigrate a specific job – to be used if someone downgraded from 1.597 to a lower version. This tool was removed last year

https://github.com/jenkinsci/jenkins/pull/9092

Now that it has been 10 years since this change, and it is very much likely that no one uses 1.596 or below version anymore, we can safely delete the RunIdMigrator.