そういえばPortal要素ってどうなったの？

The Portals proposal is not currently being actively pursued.

Many of the capabilities that Portals would have enabled are being explored in other proposals:

• navigation speculation (including speculation rules, prefetch, and prerendering)
• View Transitions
• Fenced Frame
• and more (which may be added here)

This repository is thus effectively archived for the time being.

Portal要素自体は止まっていて、構成要素を複数の仕様に分割してそっちを進めているっぽい。