In a previous discussion , it has been clarified that it is not possible to create and enable an AppVM-specific systemd unit without modifying the template too.
Well we are creating Templates for the ProxyVMs, so it is still more proper to use systemd services instead of manually adding to /rw/config/rc.local.
not possible to create and enable an AppVM-specific systemd unit
Somehow I doubt that. systemd can use non standard locations for units, and bind-dirs exist.
there is no document which says that /rw/config/rc.local is deprecated, legacy or discouraged to use in Qubes OS.
That may be the case, but it is still very bad practice. It works in the same manner as the old rc.local and comes with all of its drawbacks. There is a reason why traditional distros deprecated this stuff, so I don’t think it should be encouraged at all.