We generally focus on only the newest version of Spine, as there are many old versions and it would be too much effort to go back and apply fixes to them all. Spine 3.4 was released 9 years ago!
You may be able to export to JSON, fix the problem in a text editor with find and replace, then import again.
There are hotkeys to move the selection in the tree. It might not be so bad to only show bones in the tree, then move the selection down while watching the checkbox -- check it when you see it unchecked. Though I'm not sure if we had these hotkeys 9 years ago.
