Thanks a lot for the pointer. Your phrasing seems a bit misleading, though. A manylinux201x-compliant wheel can actually use whatever setting it wants for that variable, because the compiler in the official build image will simply ignore the setting and use the old ABI.
The question asked here is a bit different, though. Imagine one circumvents the Redhat/CentOS issue that prevents from using that flag, which value may be preferred, and are there any runtime issues with either?
These have to do with a non-standard usage of wheels, though: when people try to link their own C++ code against the C++ libraries shipped in a binary wheel. While it is nice to be able to do this, it’s certainly a non-goal for the manylinux specs.