Another user-visible difference:

$ ls /tmp/py37/bin/python*
/tmp/py37/bin/python3    /tmp/py37/bin/python3.7-config  /tmp/py37/bin/python3.7m-config
/tmp/py37/bin/python3.7  /tmp/py37/bin/python3.7m        /tmp/py37/bin/python3-config
$ ls /tmp/py38/bin/python*
/tmp/py38/bin/python3    /tmp/py38/bin/python3.8-config
/tmp/py38/bin/python3.8  /tmp/py38/bin/python3-config

Granted, these differences aren't the end of the world but:

"But I'm not sure of what are the things impacting by ABIFLAGS="" rather than ABIFLAGS="m"."

Not to beat a dead horse but - we really should not make changes like this without first understanding what the impact will be.  As it stands, this change potentially affects end-users, third-party distributors of Python, and package developers.  And, for what reason?  I don't wsee how this change provides any real value for anyone and it does cause people to have to change things, like creating conditional code for 3.7- and 3.8+.  If we were starting from scratch today, sure - "m" is not needed.  But just because we can change something doesn't mean we should.