For the foreseeable future, you will still need to download any mods to play on custom-modded servers for Arma 3 outside of the game, as integrating Steam Workshop with Arma 3 servers has numerous technical obstacles, Bohemia has confirmed.
If you are a modder for Arma 3 and run your own server, you will know the difficulty in attracting players to your server. Currently, if you run mods on your server, such as the now-defunct Arma 3 Life mod for example, anyone who wishes to play will have to download that mod via a third-party or from the Steam Workshop so that you have the same files the server does. With Steam Workshop integration for Arma 3 making modding more accessible than ever and easier to share your items with the Arma 3 community, it looks unlikely that Bohemia will make the Steam Workshop integrate with Arma 3 servers.
It may not seem like a big deal to most Arma 3 players, however it would be a huge step forward for Arma 3 modders if the Steam Workshop became integrated with servers. Basically, when you join any server for Arma 3, you will download the mission file, but if yo want to play on a modded server you must already have the mod installed on your computer. However if this integration was added, you would be able to download the mod straight from the server, the same way you would the mission file. This would allow for you to then play on modded servers without ever having to exit the game to download the mod.
If you played Island Life on Arma 2, you will know how much of a pain it was to install the mod. Wouldn’t it be great if you no longer needed to do that?
Unfortunately for modders, there are obstacles to overcome for Bohemia which make it unlikely to ever happen.
When asked on Twitter if they planned on integrating the Steam Workshop with Arma 3 servers, Bohemia said there were technical obstacles, meaning that they weren’t sure at the moment.
@VGCUK It’s not sure at the moment. There are also some technical obstacles, such as servers not requiring Steam ownership.
— Arma 3 (@Arma3official) July 13, 2015
Anyone who has played on a Garry’s Mod server will understand how much easier it would make playing on a modded server if it was added as you also download the mod when you join the server on Garry’s Mod.
Hopefully Bohemia will find a way to overcome the obstacles and add in Steam Workshop integration with Arma 3 servers as it would no doubt lead to more feature-rich and more entertaining servers to join, as well as more players for those communities which currently run modded servers.