No like, the point is that the only reason you (and I: I do this all the time, including with my open source software... like: no judgment) want to target some

No like, the point is that the only reason you (and I: I do this all the time, including with my open source software... like: no judgment) want to target some old version of glibc is so you can distribute that binary to people without caring as much about what version of the OS they have; but that would be unnecessary if you just gave them the source code and have them compile their own copy for their system targeting the exact libraries they have.