The problem isn't new symbols, it's that existing symbols can get the new version depending on what version you compile it against. On my glibc ~200 symbols have the newest version (2.34) but these are largely all methods that have been around forever. A binary built here won't run in most places.
You seem to want some way to have your code run on any platform whatever features you use in it. That you can't have. Of course if you want to support CentOS 6, you have to avoid any new symbol.