There have been use-cases where the developed product got split into different version due to breaking changes, but both versions would need to be actively maintained. (ex. the Angular framework with AngularJS and Angular 2)
In such a case, there are 2 approaches, you can either fork the project into a second one and maintain the two versions as two completely separated projects, or you can maintain them in the same project.
For the second option, SwatFlow supports 'lts/' long term support branches, which allow active development on other branches than develop.
Long term support branches should only be maintained if actual development is planned, for maintaining a release and providing fixes, hotfixes should be able to satisfy any requirement.

Once the lts branch separation occurs, there should not be any more contact with develop or other lts branches.