Steps to create unsigned IPA (Tested on Xcode 9.4.1)
Step 1: Open finder > Go to Folder.. as below screen

and then copy and past the below line:
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS*.*.sdk/SDKSettings.plist
Open iPhoneOS.sdk as showing in below image:

Step 2:
Copy the SDKSettings plist in another folder because you can't make changes here:
Step 3: Make change in duplicate
set CODE_SIGNING_REQUIRED to NO
Step 4: Now replace duplicate Plist with original one (Both name must be same).
This will also ask admin permission to change.
Warning: Please make sure you have copied the original plist and save somewhere for future reference.
Step 5:
Now set code signing identity to as below image:

and now you can create the Archive file

**Step 6:**Right click on this test Archive file and show in finder:
Again right click and click on show package contents
then Products>Applications>and copy the .App file
Step 7:
- Copy the .app file into a new subfolder named Payload (this folder
name is case sensitive)
- press Payload folder and rename it to app_name-version_number.ipa
- Example- Test-version_1.1.ipa
And finally you have unsigned IPA file :)
Note: After done with your app changes replace the original plist again or set set CODE_SIGNING_REQUIRED to YES