Just because qubes hasn’t deprecated it doesn’t mean that other distros have not done it.
https://suckless.org/sucks/systemd/
Except its functionally is the same and the implementation is practically the same - execute /etc/rc.local late in the boot process
From the look of it, it is running after network-pre.target, qubes-mount-dirs.service, qubes-network.service, and qubes-firewall.service.
If you want to run something before network.target, put it in /rw/config/qubes-firewall.d. See the doc @renehoj linked to.
The qubes-mount-dirs.service stuff is unavoidable, but it sill would be nice to be able to run other stuff before the other 3. qubes-mount-dirs runs as early in the boot process as it possibly can, before other mount points like rw.mount, home.mount and even the gui-agent, so if it is possible to launch custom services this early it will be nicer.
You can suggest it on GitHub and post a link to it here.