There should be a warning that flags a feature class when you are trying to add it to a branch-versioned feature dataset. I am in 2.7 currently and just dealt with support for weeks for editing errors with no documentation online because I added a feature class that was unversioned to a versioned feature dataset. Feature classes that were already being edited through branch versioning no longer could be edited and it took weeks to get to the bottom of the issue. The flag I'm proposing could have an option to version the new feature class that's being added to the feature dataset exactly as the rest of feature classes so that these errors don't occur!