That swagger path hasn't been updated for the unified UI yet; but if you put a ticket in with GCP support or ask someone from your customer success team they should be able to get your tenants response platform url that will work.
Hello,
If you know the SOAR URL(siemplify-soar.com) portion of your SecOps instance adding "swagger/index.html" still does work. If you don't please put in a ticket to SecOps Support and they can provide it for you.
Pro-tip from a SecOps user that had to find this as well: If you create a temporary webhook in the SOAR, it will tell you your URL 🎉

-mike
Hello,
If you know the SOAR URL(siemplify-soar.com) portion of your SecOps instance adding "swagger/index.html" still does work. If you don't please put in a ticket to SecOps Support and they can provide it for you.
You can find the SOAR URL in any SOAR settings by looking at the URL to which API requests are being sent in the DevTools of your browser.

and then as @TonyH mentioned above, you can add to it "swagger/index.html" to load swagger:

Thank you guys.
Both @f3rz and @mikewilusz 's answers can find soar URL.