Excerpt from discord discussion

so you'd have to check for "user owns DLC, and has either GM or compat pack loaded"

Dahlgren
@dedmen if you join a server running GM DLC, load GM DLC if owned and if not, prompt to subscribe to GM compat and load that instead

Dedmen
A server having GM loaded might switch to weferlingen at any time. At which point you do need to own it

And a owner doesn't have any reason to load the compat instead of the official.

SK
yeah, you can connect to a regular arma server without tanoa or livionia, and you'll be kicked from the server if a tanoa/livionia mission is loaded

It might be bad UX if the Launcher says "Yeah you're golden with the compat" but then the server switches to weferlingen and they can't play.
The Launcher can detect if server is currently running DLC terrain and require ownership in that case, which might already be the case.

The launcher shows "Get DLCs" if Tanoa is active. So yes its already supported.

I could indeed do some hackery to remove the missing DLC block, and just force add the compat mod to the servers required mods.
But if the server is running CDLC terrain, that CDLC isn't actually optional so compat mod wouldn't work.

Also even if I check the current running terrain, the server running CDLC can always switch to its terrain.
If I added it, people would expect it to work, which it won't.

We would tell people "yeah you can download this 20+GB compat data mod and join the server just fine", then they download it, try to join, and get an error because they can't play on the terrain.

We would need a way to tell that a server guarantees that despite having CDLC data loaded, it will never switch to CDLC terrain.
Which I don't see a sensible way to do.

I feel this defeats the purpose of the launcher, as it serves as a great tool to help guide people into loading the correct mods.

Yep, the purpose for the launcher is a simple one-click and everything will be fine and working solution. But it just doesn't work like that here.