Backups can be made using oandbackup or
If you created a backup of system applications before switching from the factory image or a different Android distribution to Replicant or before an upgrade to a new major release (e.g. from Replicant 4.2 to Replicant 6.0), restoring this backup will cause issues. The installation pages require a factory reset in these cases because the data is incompatible, so a backup of the data is incompatible as well.
SMS and contacts apps usually provide ways to export contacts and messages. Using these means to backup and restore the data will likely be successful and won't result in misbehaving apps.
Another way to backup application data is to synchronize them with a remote server.Several applications can do that:
- Run the remote server software yourself on a server. For instance Parabola has a nextcloud pacakge , so you could install Parabola on your server and use that. PureOS probably has a freedombox package, so you might be able to install some software there too.
- Use a provider. For instance the CHATONS project provides a charter and list hosters abiding to that charter. The charter is meant to respect users (so it for instance forbid selling users data and require hosters to use free software). The librehosters also aims to do something similar.