Despite making the voluntary decision to leave their royal titles behind, in reality, it is only in name. Harry and Meghan are still inseparable from their royal status, and still members of the royal family. Therefore, their security needs and risks have remained unchanged.
The only thing that has changed is that they will no longer participate in royal duties, and will not directly receive public funds for their use. Their security costs may be inconvenient considering their departure from royal duties, but it is necessary considering that they still retain the public image that is impossible to leave behind.