Hello, it's me again! I'm currently in the process of making a port to 1.18.1 when I stumbled across a roadblock. 

package com.Polarice3.FireNBlood.items.capability;

import com.Polarice3.FireNBlood.items.handler.SoulUsingItemHandler;
import net.minecraft.core.Direction;
import net.minecraft.nbt.Tag;
import net.minecraft.world.item.ItemStack;
import net.minecraftforge.common.capabilities.Capability;
import net.minecraftforge.common.capabilities.ICapabilitySerializable;
import net.minecraftforge.common.util.LazyOptional;
import net.minecraftforge.items.CapabilityItemHandler;
import net.minecraftforge.items.IItemHandler;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

public class SoulUsingItemCapability implements ICapabilitySerializable<Tag> {
    private final ItemStack stack;
    private final LazyOptional<IItemHandler> holder = LazyOptional.of(this::getHandler);
    private SoulUsingItemHandler handler;

    public SoulUsingItemCapability(ItemStack stack) {
        this.stack = stack;
    }

    @Nonnull
    private SoulUsingItemHandler getHandler() {
        if (handler == null) {
            handler = new SoulUsingItemHandler(stack);
        }
        return handler;
    }

    @Nonnull
    @Override
    public <T> LazyOptional<T> getCapability(@Nonnull Capability<T> cap, @Nullable Direction side) {
        return cap == CapabilityItemHandler.ITEM_HANDLER_CAPABILITY ? holder.cast() : LazyOptional.empty();
    }

    public Tag serializeNBT() {
        return CapabilityItemHandler.ITEM_HANDLER_CAPABILITY.writeNBT(getHandler(), null);
    }

    public void deserializeNBT(Tag nbt) {
        CapabilityItemHandler.ITEM_HANDLER_CAPABILITY.readNBT(getHandler(), null, nbt);
    }
}

In 1.18.1, the write and readNBTs are now obsolete so now I've no clue on how to properly set up my Capability again. Any suggestions?

add a serialize and deserialize method to your CapabilityHandler call there the logic from write and read method, then call these two methods from serializeNBT and deserializeNBT of your CapabilityProvider. Note you can call the logic from the read and write method directly from your CapabilityProvider if you hava setters to set the values of your Capability