A method for setting NBT-Tags for items in spigot. Please note, the NBT-Tag with the path "test.helloworld" (for example) will look like this: {test:{helloworld:<data>}}

NBTEditor.java

public class NBTEditor {
        public static ItemStack setNBTValue(ItemStack is, String path, String value) {
            path = "null." + path;
            String[] pathArray = path.split("\\.");
            pathArray[0] = null;
            net.minecraft.server.v1_16_R3.ItemStack nms = CraftItemStack.asNMSCopy(is);
            NBTTagCompound[] nbts = new NBTTagCompound[pathArray.length - 1];
            for (int i = 0; i < pathArray.length; i++) {
                String part = pathArray[i];
                if (i == 0) {
                    nbts[i] = nms.getOrCreateTag();
                } else {
                    if (i == pathArray.length - 1) {
                        nbts[i - 1].setString(part, (value == null) ? "" : value);
                    } else {
                        nbts[i] = nbts[i - 1].getCompound(pathArray[i]);
                    }
                }
            }
            for (int i = nbts.length; i-- > 0; ) {
                if (i - 1 < nbts.length && i - 1 >= 0) {
                    nbts[i - 1].set(pathArray[i], nbts[i]);
                }
            }
            nms.setTag(nbts[0]);
            return CraftItemStack.asBukkitCopy(nms);
        }
    }