> Otherwise, why use the system libmpdec at all and not the version
shipped with Python?

the packages are faithful reproductions of upstream packages, deviating from those introduces surprises for downstreams


> If I install into a venv, I also don't use the system libmpdec.

that's simply not true:

$ python3.9 -m venv vvv
$ vvv/bin/python -c 'import _decimal, subprocess; subprocess.check_call(("ldd", _decimal.__file__))'
	linux-vdso.so.1 (0x00007fff429ec000)
	libmpdec.so.2 => /lib/x86_64-linux-gnu/libmpdec.so.2 (0x00007fcaeae03000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fcaeade0000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fcaeabee000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fcaeaa9f000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fcaeae7f000)