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The "addonpacks" folder is not a general feature of Forge/Fabric. It’s just one specific mod that choose this very poor name for the folder that should contain its addons.
Whether mod addons can be dangerous depends on the mod the addons are for, and there is no general system for loading addons.
If a mod has a safe addon system, we usually allow the upload of addons for that mod. It is, for example, possible to upload Lucky Block addons or resource packs for that weird train mod.
The issue with the addonpacks folder in your case is that while the mod you are trying to install addons for might be safe, other unsafe mods might use the same, very generic, folder name, which is why we can’t allow uploads there. Mods should either use a folder name that contains the mod’s name or just put their addon folders inside their config folders, so that there can’t be any collisions between addon locations.
For the reasons explained above, there is no option to add files to the addonpacks folder.
If a modpack already includes the addons by default it would work. Adding custom addons would not be possible in modpacks either.
