There are a few backup possibilities.
Most cloud providers offer some form of backups through snapshots. Digital Ocean allows to do snapshot the volumes, which is MongoDB, Mysql and Images.
More info on backups with Digital Ocean is found here: https://www.digitalocean.com/docs/kubernetes/how-to/snapshot-volumes/
We have 2 cronjobs in the API for backing up MongoDB en Mysql that can connect to an S3 storage bucket. Both AWS as Digital Ocean support S3. For both providers rules about expiration and deleting old files can be set through their API. Openstad currectly doesn't actively manage removing old backup files.
These get activated if certain environment values are set in the Helm values, mostly managed by a custom-values.yml.
There is also a shell script to be found that can connect to a Openstack storage, although this one is not added to a cron and has to be added manually.
(We advise to backup at least Mysql and Mongodb also to an external provider.)