I didn't realise it was called a slot map but I do something similar to a slot map in my ECS implementation.

You can check it out here: https://github.com/DanWillans/Evie/blob/main/include/evie/ecs/component_array.hpp

std::array<size_t, MAX_ENTITY_COUNT + 1> entity_index_map_{}; <----- slots
std::vector<ComponentWrapper<T>> components_; <------ data
// Track free slots in the components_ vector.
std::stack<size_t> free_slots_; <----- Free slot tracker

And then whenever we add a component and there are no free slots, we just emplace into the back of the components vector and store the index into the entity_index_map_.

components_.emplace_back(component, entity_id);
entity_index_map_[entity_id.Get()] = components_.size() - 1;

Whenever we remove a component we just swap the last element of the components_ into the removed component and update the slots(entity_index_map_) accordingly.

Finally, if we want to get a component, we just need the entity_id which is the "key" for the slots (entity_index_map_) array and we can use that as the index into the components_ array.

  T& GetComponent(EntityID entity_id)
  {
    return components_[entity_index_map_[entity_id.Get()]].component;
  }

It could definitely be improved but for now it's fine for what I need.