This plugin for IB3 essentially removes all traces of GenericToolbar from a nib file. The first time you load the nib, the toolbars will be replaced by dummy instances which are then encoded as plain NSToolbars when you save and close the file. All connections should be preserved.
If you want to retain instances of GenericToolbar in your nib file, only edit the file using Interface Builder 2 and the older palette version of GenericToolbar.
You can safely ignore the plugin incompatibility warning when loading a GenericToolbar-containing nib file.
IB3's toolbar items do not support duplicates or the enabled/disabled checkbox option (bindings should still work but are untested).
GenericToolbar supported multiple windows sharing the same toolbar through a special type of outlet connector. IB3 does not support this, so you have a choice of how to resolve the issue (under the plugin preferences for GenericToolbar).
If there is still demand for the features of GenericToolbar that are missing from IB3, or if there is a very strong demand for the ability to share toolbar-containing files between IB2 and IB3, GenericToolbar development may continue. Otherwise, the project will simply fade away, now that Apple has finally figured out that toolbars are an important part of interface design.