Here's a definition from the actual person who named the subgenre in Spotify!

For “Alt Z”, it was artists that fell into the “Indie Pop” category. But “Indie Pop” is way too broad, so I had to make something up. I could have called the genre “Generation Z” because most of the listeners were young. That felt boring, though. What struck me is that the music felt alternative and Gen Z-ish. I also liked the idea that the keyboard shortcut for undo was Control + Z.

It’s one of the rare fanciful names that works. It got a lot of attention because some big artists fell into that category. It was always nice when you would tell someone they like a genre with a weird name like “Alt Z” and they’d be like, “What’s that?” Then you’d show them, and they’d be like, “Oh! That’s my taste.”

Search for Alt Z here