How can the app's owner allow past purchases before 2016 to share the app in Family Library?

From:
https://support.google.com/googleplay/answer/7007852

• Apps & games sharing with Family Library may be limited in some countries.
• You can't share in-app purchases and free apps with your family members.
• Any apps or games purchased after July 2, 2016 are eligible to be added to Family Library. If you purchased the app or game before July 2, 2016, it’s eligible for Family Library if the developer has made past purchases available. You can find out whether this is the case by tapping Read More on the app’s details page.

So the developer has to allow sharing if bought earlier.

 

I found this, but it's from four years ago so things could have changed:

1. Sign in to your Google Play Developer Console.
2. Select an app.
3. Select Pricing & Distribution.
4. In the "Family Library" section, select the checkbox.

I'll move this to the Dev section