I used following steps on CentOS-7 and they worked fine for me:

yum install centos-release-scl
yum install devtoolset-10
scl enable devtoolset-10 bash

I used same flags which you used:

g++ -std=c++2a -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/dummy2.d" -MT"src/dummy2.o" -o "src/dummy2.o" "../src/dummy2.cpp"

devtoolset-10

Every g++ comes with it's own headers. /usr/include/c++/4.8* is for 4.8.5 only.

devtoolset-10: g++ version 10 is using the headers at /opt/rh/devtoolset-10/root/usr/include/c++/10

"enable" libstdc++-10

There is no shared library "libstdc++-10". There is /opt/rh/devtoolset-10/root/usr/lib/gcc/x86_64-redhat-linux/10/{ libstdc++.a, libstdc++.so } , where libstdc++.so is a ~200B text file.

Developer Toolset uses a hybrid linkage model. Programs built using it use the system libstdc++ library as far as possible. Missing bits (those that were added in later C++ versions) are automatically supplied via static linking.

As a result, Developer Toolset can be used to build applications using newer GCC versions. But it cannot be used to run applications which have been built on other distributions, with newer GCC versions (assuming that is what you are trying to do).