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Use the overload of that method that takes in an ItemStack as one of it's parameters.

This happens because you are extending ItemSword and instantinating the base MultiMap through a super invocation. That returns you a multimap with the sword's base values already in place set by ItemSword. Your additions are not taken by the game since they are already applied by ItemSword, and they don't override the old ones because it's a MultiMap. Basically create a new empty multimap instead of grabbing the one from ItemSword.

I've pondered this issue too, and for now, I am content to spread my gaming time a bit more broadly. I tried Skyrim SE (The Elder Scrolls) for the first time, a week or so ago. Interesting to me, that game ALSO has a bit of diversity in how people mod it and run it. Several different "mod organizers" are in use in that gaming tribe. Like @vpontin, I am mostly happy to continue on 1.12 Minecraft games while we wait for a new Forge update. (My two young grandsons casually play 1.13, but they are not seriously doing a lot there, so I get a glimpse when I visit them, of the updates. ) I do admit that checking on the status of work on Forge keeps me looking in here, and staying abreast of things MineCraft! Neither game is all that bad played Vanilla, IMO. However, both do look a lot nicer with mods and some of the things mods can let a player try, are worth the wait.

Why use it? -It can pull any object from any registry, during runtime. Including objects from other mods. Why is it useful? -Let's say I have a mod called "SuperMod", and you want to use one of my blocks, but don't want to use my mod as a dependency. Instead, you can use ObjectHolder to "get" this block at runtime, and thus use it for whatever you want, without having a dependency on my mod. Of course, it is always good to only run this code if "SuperMod" is also loaded. Otherwise, it is going to stay null. (Use Loader.isModLoaded("modid") to check!)