No IPv6 Address Due to macOS

Then have a few suggestions ;

1 - Get Support Choose a product and we’ll find you the best solution.Start now and open an Apple Support Ticket as they are Apple Employees to deal will these types of issues . 

2 - Product Feedback - Apple and make it known to Apple regarding this ongoing issue 

3 - Have the computer evaluated by the Professionals who have the Special Hardware / Software and Expertise to diagnose the issue and offer possible solutions

macOS100 wrote:

I have an iMac running 13.1. My ISP does provide IPv6 via PPPoE, and it does work when I use a router. However, when I connect directly using PPPoE, my Mac only gets an IPv4 address and no IPv6. I know this problem already existed for a long time, but I wonder if anyone has a good solution.

It sounds from your description that your ISP requires a gateway or router to be connected to their service. Contact your ISP and ask them the requirements for connecting to their service.

Is the supplied equipment from the ISP capable of " IPv6 Addresses "

That is something to Ask the ISP.

Say it does does not make it so.

macOS100 wrote:

I believe it's a problem of macOS.

I believe it is a problem with how you are connecting to your ISP.