What are outdated templates and why do I need to revert them? Can't I just continue using the old ones?
If a template is changed between releases, i.e. from 1.0.0 to 1.0.1, and you have manually edited it, then it will be marked as outdated. This is done because the software doesn't automatically overwrite those templates, thereby causing certain functionality which you have added to be lost. It is an indication that the core code has changed and you need to take action. The software will continue to function with these outdated templates but you may be missing important updates and bug fixes so it is recommended to revert and reapply any edits.
It seems the best way to handle this is to create a child style and only every apply your edits on that. If something changes in the original style, you can just update it but your changes to the child style file should stay intact.
There might be some cleaning up to do (say if you edited something in the child that then changes when the parent is updated) but if there is you at least still have the child style with all the changes and can easily create a new child style and re-apply them.
Sorry what I meant was that when your parent style is updated it can be quite difficult to reapply all of the edits. However, if you make the edits in a child style, they should all still exist after you update the parent and you won't need to revert and reapply them unless they specifically affected bits in the parent that have changed.