Well, S/4 still sits on NetWeaver (even the cloud version) so unless they've taken CALL TRANSACTION out of NW 7.50 ABAP then BDC is still there. And given that also ERP EHP 8 is NW 7.50, I don't believe they could do something so radical. So in the on-prem version where you can still write ABAP if you want, BDC is alive and well. At least, in my model of the Universe 
Again, I'll find out in a few weeks!
The cloud version is, of course, a different matter entirely, since you can't write your own arbitrary ABAP...
Steve.