This is a quick guide to getting Photo Mode to work in Half-Life Alyx. There may be a better way to do this - let me know if there is.
The photo mode button is commented out in the game data files. You can use a hex editor to remove the comment tags and have the photo mode button display in the game menu.
This video shows the menu in the game: • Half Life Alyx Photo Mode
So I think I've worked out how to use it. Move the smaller panel around to where you want to take a picture then press F12. Skyboxes are messed up and the camera frame leaves a shadow behind. Images here: https://m.imgur.com/gallery/SpzFto7