macOS Won't Show WiFi Login Page
On macOS Catalina (10.15.1), the solution for me was to manually open the /System/Library/CoreServices/Captive Network Assistant.app application in the Finder, which will trigger the standard application for displaying the WiFi login portal.

You might still need to follow the suggestions listed above in order for this to be successful (since I had completed them prior to attempting this solution).

I found this solution from a blog post that has a more in depth solution for automatically triggering this process automatically until Apple releases a fix. But since it's not that difficult to manually open the /System/Library/CoreServices/Captive Network Assistant.app application I decided not to go through the entire workaround process.



Edit: it's important to note that not all networks will require this solution. In some cases just navigating to a non SSL site (examples provided in original question), will work just fine and show the login page. But if you get an error message about not being connected to the internet, this is a good thing to try.