Last Update: May 13, 2019
Offline Version
List of oldest supported version of top 10 Linux Distros and their glibc version according to distrowatch.com.
Out of all versions with published EOLs, 2.12 is the oldest glibc still active, found in CentOS 6.8.
If CentOS 6 and 7 are eliminated, the oldest glibc is 2.23 in Ubuntu and Slackware.