I think it's also possible to tell Syncthing to sync file adds/updates but not deletes. I wouldn't trust it 100% with my files, though, probably still a good idea to manually take regular backups to an external hard drive or something.

IIRC, sync thing has an option for a “trash folder” where deleted files after syncing can go. It’s not quite full backup, but it also lets you recover files for some period of time.

Edit: actually it’s the .stversions that I was thinking of. Deleted versions can go there.