At the core of this agreement is Apple’s definition of a “reader” app:

Reader apps provide previously purchased content or content subscriptions for digital magazines, newspapers, books, audio, music, and video.

And:

Because developers of reader apps do not offer in-app digital goods and services for purchase, Apple agreed with the JFTC to let developers of these apps share a single link to their website to help users set up and manage their account.

Apple’s announcement:

Apple today announced an update coming to the App Store that closes an investigation by the Japan Fair Trade Commission (JFTC). The update will allow developers of “reader” apps to include an in-app link to their website for users to set up or manage an account. While the agreement was made with the JFTC, Apple will apply this change globally to all reader apps on the store.

The change goes into effect in 2022. Feels like Apple is breaking down the overall App Store structure so they can make changes to individual categories to address the wave of anti-trust scrutiny/legislation.