Maybe I didn't explain my use case clearly.
Yes, I can extract all the metadata I need from the game page itself. The problem is that I want to display that information on my extension's settings page, which is separate from itch.io.
That means I either have to cache all of the game's metadata locally (which significantly increases the amount of stored data and requires a mechanism to keep it up to date), or store only an identifier and fetch the rest when needed.
I think the second approach is preferable. In fact, I can already do it using URLs like https://username.itch.io/game-slug/data.json. What I don't like is that this requires storing the author's username and the game's slug instead of a single immutable ID.
Besides taking up more space, usernames and slugs can change over time, while the game's ID remains constant.