gcc LTO appears to strip debugging symbols

The situation should have improved by now. GCC 8 finally got the early debug info improvements: http://hubicka.blogspot.com/2018/06/gcc-8-link-time-and-interprocedural.html

While it was possible to build with LTO and -g and debug the resulting binary, the debug information was kind of messed up C, instead of debug info corresponding to the language program was originally written in. This is finally solved. [...] The main idea is to produce DWARF early during compilation, store it into object files and during link-time just copy necessary fragments to final object files without need for compiler to parse it and update it.

But note that -gsplit-dwarf won't work with LTO.

It's because gcc does not support combine -flto with -g.

You can find the details GCC Online Docs - Optimize Options

"Combining -flto with -g is currently experimental and expected to produce unexpected results."

When you use -flto, the -g will be ignored.