You only need to worry about ABIs when you are crossing library boundaries. Within your own applications/libraries the ABI doesn’t really matter as presumably all your object files are compiled with the same compiler version and switches.
If you have a library compiled with ABI1 and an application compiled with ABI2 then the application will crash when it tries to call functions from the library as it wont pass the arguments correctly. To fix the crash you would need to recompile the library (and any other libraries it depends on) with ABI2.
In your specific case as long as you compile nlohmann with the same compiler version as your application (or are just using nlohmann as a header) then you don’t need to worry about the ABI change.
Globally suppressing the warning seems to be a dangerous option as it will prevent you from seeing any future ABI issues. A better option would be to use #pragma to disable the warning just for the functions in question, e.g.:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wno-psabi"
void foo()
{
}
#pragma GCC diagnostic pop