Specific changes to glibc 2.28 that affect PostgreSQL
I'm not sure what the minor change were but even minor changes affect how the index stores data on disk, any difference would result in a collision and/or corruption when using a index that had the old sort method.  data is could be looked-up and/or stored in the wrong places..   

This requires drop and rebuilding of the index
The source code to glibc can be found here

https://packages.debian.org/buster/glibc-source

Postgresql did add a new feature I think in 10 ICU
which allows the creation of custom Collation tables.
https://www.2ndquadrant.com/en/blog/icu-support-postgresql-10/ 

Which could be used to avoid these problems in the future