Model

class Contact include ActiveModel::Model
  attr_accessor ..., :confirm   # confirmを追加
  ...
  validates_acceptance_of :confirm, allow_nil: false
end

View

<div class="form-check">
  <%= form.check_box :confirm, class: 'custom-control-input' %>
  <%= form.label :confirm, '上記の内容で送信します。よろしければチェックを入れてください。', for: 'contact_confirm', class: 'custom-control-label' %>
</div>

Controller

def sending
  params[:contact][:confirm] = ActiveRecord::Type::Boolean.new.cast(params[:contact][:confirm])
  @contact = Contact.new(contact_params)
  if @contact.valid?
    ContactMailer.send_mail(@contact).deliver_now
    redirect_to contact_done_path
  else
    render :index
  end

  private
    def contact_params
      params.require(:contact).permit(..., :confirm)   # confirmを追加
    end
end