Can I create an encrypted Time Machine backup on a SoftRAID volume?

Yes, Time Machine supports encrypted backups on SoftRAID APFS volumes.

Important setup requirement: When creating your Time Machine volume, select APFS as the file system. macOS does not support converting HFS+ volumes to APFS, so if you initially created an HFS+ volume, you must delete it and recreate it as APFS before using it for Time Machine.

Encryption performance limitation: RAID 4/5 volumes on HDDs (rotating media) are currently unacceptably slow when encrypted. We strongly recommend that you do NOT encrypt RAID 4/5 volumes on HDDs. RAID 0 and RAID 1+0 volumes work fine with encryption on all drive types. Flash media (SSDs/NVMe) performs well with encryption regardless of RAID type.

When you configure Time Machine, macOS will prompt you to encrypt your backup volume. Only enable encryption if you are using RAID 0 or RAID 1+0.