insmod and modprobe are still synchronous. Once you have loaded a network driver, you can be assured the corresponding devices have been created.

What's new (around 2006) is the kernel will probe buses in parallel and invoke modprobe in parallel to load the appropriate drivers if they match the driver's alias.

Introduction of userland components to populate /dev also largely predates systemd. devfs was introduced during 2.3.x. udev replaced it during 2.5.x.