When logging the deletion of a user, don't store its id in the target_user_id column. It will never match a user record.

app/services/staff_action_logger.rb

@@ -9,7 +9,6 @@ class StaffActionLogger
     raise Discourse::InvalidParameters.new('user is nil') unless deleted_user and deleted_user.is_a?(User)
     UserHistory.create( params(opts).merge({
       action: UserHistory.actions[:delete_user],
-      target_user_id: deleted_user.id,
       email: deleted_user.email,
       ip_address: deleted_user.ip_address.to_s,
       details: [:id, :username, :name, :created_at, :trust_level, :last_seen_at, :last_emailed_at].map { |x| "#{x}: #{deleted_user.send(x)}" }.join(', ')

spec/services/staff_action_logger_spec.rb

@@ -28,9 +28,8 @@ describe StaffActionLogger do
       expect { logger.log_user_deletion(1) }.to raise_error(Discourse::InvalidParameters)
     end
 
-    it 'creates a new StaffActionLog record' do
+    it 'creates a new UserHistory record' do
       expect { log_user_deletion }.to change { UserHistory.count }.by(1)
-      UserHistory.last.target_user_id.should == deleted_user.id
     end
   end
 
@@ -56,7 +55,7 @@ describe StaffActionLogger do
       expect { logger.log_trust_level_change(user, old_trust_level, max_level + 1) }.to raise_error(Discourse::InvalidParameters)
     end
 
-    it 'creates a new StaffActionLog record' do
+    it 'creates a new UserHistory record' do
       expect { log_trust_level_change }.to change { UserHistory.count }.by(1)
       UserHistory.last.details.should include "new trust level: #{new_trust_level}"
     end
@@ -69,7 +68,7 @@ describe StaffActionLogger do
       expect { logger.log_site_setting_change('abc', '1', '2') }.to raise_error(Discourse::InvalidParameters)
     end
 
-    it "creates a new StaffActionLog record" do
+    it "creates a new UserHistory record" do
       expect { logger.log_site_setting_change('title', 'Discourse', 'My Site') }.to change { UserHistory.count }.by(1)
     end
   end
@@ -106,7 +105,7 @@ describe StaffActionLogger do
       expect { logger.log_site_customization_destroy(nil) }.to raise_error(Discourse::InvalidParameters)
     end
 
-    it "creates a new StaffActionLog record" do
+    it "creates a new UserHistory record" do
       site_customization = SiteCustomization.new(name: 'Banana', stylesheet: "body {color: yellow;}", header: "h1 {color: brown;}")
       log_record = logger.log_site_customization_destroy(site_customization)
       log_record.previous_value.should be_present