Android 16 is still months away from release, but is nearing feature freeze.
They will introduce an OPT-IN permission for asking user local network access. Opt-in is opt-in for developers, meaning apps that don’t use this permission will still get access to local network.
Hopefully, Graphene OS will allow some kind of restriction like the Network toggle. However, contrary to the network permission, apps wouldn’t have to declare it in their manifest, so blocking access would be a custom integration in the networking stack.
These restrictions are implemented deep in the networking stack, and thus they apply to all networking APIs. This includes sockets created in the platform or managed code, networking libraries like Cronet and OkHttp, and any APIs implemented on top of those. Trying to resolve services on the local network (i.e. those with a .local suffix) will require local network permission.