You can only turn off Chat Notifications if you are a Premium subscriber.
If you are using the free tier, chat notifications are mandatory. We want to make sure musicians are compensated fairly. It's fine if you would rather not pay for Pretzel Premium (which supports musicians with royalties) but we simply ask you give the musicians exposure and hopefully, click-throughs.
If you're a Premium user, you can turn off Chat Notification by selecting the megaphone icon in the filters menu on the bottom left side of the player or in your settings.
In the Settings, you are able to also set the notification delay. This is useful if there is a latency delay between your stream and chat, or if you are competing in online tournaments which require a stream delay.