Is it recommended in vRA 7 to use ASD to customize the workflow stubs?
Stubs are deprecated in vRA 7, so no.
Ok. They why ASD still exist?
It isn't ASD any longer, it's XaaS. Event Broker has taken over for what stubs used to provide.
ASD is still available. Check this screenshot:-
That isn't called Advanced Service Designer, that's the Design Center and it's used to modify the IaaS models. It's only there for legacy purposes and there's no sense in using it any more. It will probably be removed in the next major version.
As per other comments, ASD is deprecated. You now should use the event broker rather than stubs.
Nothing stopping you using stubs but don't expect theM to work in future versions.