Is custom branding available for JotForm customers on all plans? If not, which plans is this feature available on?

You can set up your branding if you have a free account, add your logos on the Thank You message, Email Notifications or even display it in your form. 

But do note that there is a current JotForm branding that can be removed only on paid accounts. 

To remove it, once you have upgraded your account, go to the account settings and un-check the option to show the JotForm branding on your forms:  

https://www.jotform.com/myaccount/settings

Hope this helps.