In the teamserver overview of the project, the revisions of the delete branch are still available, including the commit message and referenced user stories/tasks.
Though the actual changes on particular documents cannot be viewed anymore. Which can only be done in studio pro.
The Studio Pro Version Control system is built on top of SVN (Until Mendix 9 where Git will be introduced :) ). You can use TortoiseSVN ( https://docs.mendix.com/refguide/using-version-control-in-studio-pro#9-working-outside-studio-pro ) to get information about all the branches and compare them in different ways (for example, see https://tortoisesvn.net/bigpicture.html).