This idea has been merged into another idea. To comment or vote on this idea, please visit CF-I-2721 Make the Curse launcher launchable offline so that games can be played where there is no internet available.
I'm not sure if this is a bug but yesterday my internet went out and I figured that I'd just play some single player modded minecraft while I waited for it to come back. Instead I was greeted with an error and locked out of the curseforge launcher. It really doesn't make sense to not be able to play the mods while offline.
Not saying offline shouldn't get added but in the meantime you can run MultiMC and add the modpacks as instances to that and play them completely offline. You'll need to export the modpacks from the launcher and make sure the export include config, mods, and resources, then you'll import from .zip in MultiMC, and then bam you'll be able to run the modpacks completely offline
Hope this helps