Regarding the argument that the / is hard to teach, there are tons of advanced features that beginners don’t need to be taught. OTOH presumably one of the problems with the current usage in Argument Clinic is that it generally produces function signatures that look just like what you can use with def, and / is the exception here. Things will be more regular once we also support it in Python syntax.
I definitely see the need for this feature, and I haven’t seen a reasonable alternative proposal, so I am strongly in support of PEP 570.