Hello,
I'm pretty sure we already have a ticket for that in Github. It's a requirement I get pretty often from the end-user.
The end-user often has a "work" space and an "archive" space. It's not a bad habit, but it causes issues with Rdiff-backup, be the end-user will move a big folder from the "work" space to the "archive" space duplicating all the file history, etc. For some users, I teach them why it's a bad habit and why they should find another way of working, but some users are resilient and keep their way of working and then complain about disk space usage of their backup...
One way to mitigate all of this would be to run --remove-older on the whole repository with a value of 365 days and then run --remove-older on the "work" space with a value of 30 days.
That would mitigate the problem of data duplication and solve the problem of disk usage.
So, where is that ticket on github ? :D