[OneFS 9.3.0.0 Release Notes]NDMP backups that are created with 9.3.0.0 work only with 9.3.0.0 and later releases.

Taka.Sさん

ご指摘はOneFS9.3のリリースノートにある以下の文言ですよね。

PowerScale OneFS 9.3.0.0 Release Notes | Dell US
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9.3.0.0 NDMP backup compatibility
NDMP backups that are created with 9.3.0.0 work only with 9.3.0.0 and later releases.

こちらはOneFS 9.3 で作成されたバックアップについての記載になるのでOneFS9.1で作成されたバックアップに関しては該当しません。
その為ご認識の通り、現在(9.1.x台)NDMPで取得しているバックアップはverup(9.5.x)後も使用(リストア)可能となります。

参考資料
PowerScale OneFS 9.5.0.0 Web Administration Guide
Page 292 
NOTE: NDMP backups that are created with PowerScale OneFS 9.5.0.0 are compatible only with 9.5.0.0 and later releases: they are not backwards compatible. The data restore path is possible from older OneFS source to a newer version of OneFS only as the target storage. Example: Restoring content from a backup created with PowerScale OneFS 9.0 to a cluster with PowerScale OneFS 9.5 is supported. However an attempt to restore content that is created from a cluster with OneFS 9.5 to a pre-OneFS 9.5 cluster is not supported.

PowerScale OneFS 9.7.0.0 Backup and Recovery Guide
Page11
NOTE: NDMP backups that are created with PowerScale OneFS 9.5.0.0 are compatible only with 9.5.0.0 and later releases: they are not backwards compatible. The data restore path is possible from older OneFS source to a newer version of OneFS only as the target storage. Example: Restoring content from a backup created with PowerScale OneFS 9.0 to a cluster with PowerScale OneFS 9.5 is supported. However an attempt to restore content that is created from a cluster with OneFS 9.5 to a pre-OneFS 9.5 cluster is not supported.

PowerScale OneFS Web Administration Guide 9.3.0.0
Page 277 
NOTE: NDMP backups that are created with OneFS 9.3.0.0 are compatible only with 9.3.0.0 and later releases: they are not backwards compatible.