Back In Time [1] is a round about 15 years old backup software using rsync in the back. I'm part of the 3rd generation maintenance team there. A lot of work in investigating and
fixing issues, understanding, documenting and refactoring old code. Have a look at Good First Issues [2]
or Help Wanted Issues [3].
[1] -- <https://github.com/bit-team/backintime>
[2] -- <https://github.com/bit-team/backintime/labels/GOOD%20FIRST%2...>
[3] -- <https://github.com/bit-team/backintime/labels/HELP-WANTED>