I recieved a 403 from your callback URL
Usong the TwitchCLI to test https://github.com/twitchdev/twitch-cli/blob/main/docs/event.md#verify-subscription
twitch event verify-subscription cheer -F https://watcher.gantzerbot.xyz/notification
✗ Invalid response. Received Forbidden as body, expected c85aee50-fff5-9ce5-2a89-b097082c477e
✗ Invalid status code. Received 403, expected a 2XX status
Looking at your NGINX config I don’t see anything in patciular wrong. It is similar to my own configs.