Does anyone use decentralized data storage for backup?

Not sure if it's quite what you're looking for but the first thing that comes to mind for me is Ceph. It's not exactly a service in and of itself but it is self hostable/open source. I currently have a cluster set up across 3 machines with 87 TiB total space and a Ceph Filesystem and thought it was relatively easy to set up (I'd recommend first doing so in a virtual environment to get your bearings if possible, though). Said filesystem is set to make 3 replicas (1 on each machine) of any data written to it and I use Unison to sync files between local storage and the cluster (such that the whole setup is analogous to Windows/Onedrive or MacOS/iCloud). I also plan on setting up a node at my parents' house and making a new replication rule for that. As they live in a different state than me, this would amount to having hot offsite backups (for both me and them). Finally, while I haven't seen it done in practice, in theory, multiple Ceph admins may be able to configure a multi-site setup where they could trade some space on their own clusters with each other for a sort of community based storage pool/hot offsite backups (like a community ran version of Google Drive or similar where the buy-in could be some of your own storage space or money). However, it's important to note that while communications are encrypted and the storage drives can be encrypted as part of setup and operation, any data written to a cluster is not automatically encrypted and if one wants privacy, said data would need to be encrypted separately before writing it to a community cluster.