It does exist, and it does work, just the docs just neglect to document it anywhere.

For those trying to do this like I just struggled to do:

1. The secret is not used for response_type=code instead you're expected to do code_challenge

• Generate a challenge, 64 long chars will do
• SHA256 hash it then encode it as base64url and send it under the key code_challenge
• set code_challenge_method to S256
• When getting your token back include the challenge under the key code_verifier

2. To get the token you need to POST to https://api.itch.io/oauth/token

3. when posting do not include the params as the body, instead it's the search query IE `https://api.itch.io/oauth/token?grant_type=authorization_code&..`