Hi Chris,
Thanks for reaching out and also for your interest in our products 🙂
Unfortunately there’s no built-in way of creating that filter. A work around would be to create an additional field with Events Calendar PRO and manually tag events that are out of stock, there’s an option to show them in Filter Bar. Not sure if that will work for you as it would require manual input of the out of stock events.
If you need to work automatically, you can always custom code it but that’s another story.
Please let me know if you have any other follow-up questions on this,
Best,
Nico