You can do this (tested on iOS):
1. Go to settings (in the Pokémon app).
2. Hold your phone in landscape.
3. Click "Report High Priority Issue" and press YES. A browser opens, and it's in landscape.
4. Close app to home screen.
5. Reopen app.
Landscape activated.
Everything besides the map gets messed up. It's usable, but the interface was clearly not designed for this.
1. Go to settings (in the Pokémon app).
2. Hold your phone in landscape.
3. Click "Report High Priority Issue" and press YES. A browser opens, and it's in landscape.
4. Close app to home screen.
5. Reopen app.
Landscape activated.
Everything besides the map gets messed up. It's usable, but the interface was clearly not designed for this.
Have fun!
I love how you can see just outside the map distance that the game renders.