The Waitlist has been replaced with a "Notify Me" feature. Select "Notify Me" on an unavailable shift and you will receive an invitation to claim it before other available Specialists if it's dropped. If the original claimer never drops the shift, you will not receive an invitation to claim it. Remember, you are expected at a shift only when it appears in the "Upcoming" section of your app.
There's no limit to the number of shifts you can get notified for.
If you still see the Waitlist option in your app, you need to update your app to the latest version. For help, see How do I update my Shiftgig app?