Long before Valve created Portal, former Nintendo programmer Giles Goddard developed a similar portal-based system and had it running in a Zelda N64 tech demo. Goddard recently shared some of the footage on Twitter after previously discussing the project on an episode of the MinnMax show in May (thanks VGC).
Goddard said the system - which he clarified he technically doesn't have the rights to - was developed as an experiment to see what the Nintendo 64 hardware was capable of.
Former Nintendo Programmer Reveals Footage Of Zelda Portals Demo
"It was basically a demo of actual portals that you could see through to other parts of the map," he said. "When I saw Portal I thought ‘oh, actually, I had that running on the N64, I should have released it then!'"
The mechanic didn't make it into the final game, and Zelda hasn't featured anything similar since.
As for the present-day Zelda games, the much-awaited The Legend of Zelda: Breath of the Wild 2 is set to be released in 2022, though we still know very little about what to expect from it, despite seeing a new trailer during E3 2021.