You can of course take a SQL dump that is version-independent, but if you're serious about creating backups, you want to take backups of the actual on-disk format of the WAL, because that's more efficient and also the only practical way to get point-in-time recovery. (For the efficiency, you could alternatively also take ZFS snapshots, which will work independently of the Postgres version, but those also don't give you PITR.) The WAL format is a Postgres implementation detail and therefore tools wanting to read and write it need maintenance whenever the format changes (which can happen on major version releases).