I get what you're saying about Steam making things trickier, but honestly, you can still run LAN missions by hosting a dedicated server on your own PC—I've done it a few times myself when I wanted to mess around with some old mods. Just go into the ArmA launcher, set up the server, and have your friend join using your IP. It works fine for most old mods, including stuff like DayZ and Namalsk Crisis. I’ve had to tweak a few things here and there, but as long as you find the right versions, you can still get it all working for a fun nostalgia trip.