I did some experimentation, and I’m pretty sure the news is good here. I installed TrueNas SCALE on a USB thumb drive (more annoying than it had to be since I had to physically remove my nvme drive so the installer wouldn’t erase it). I booted onto the thumb drive and SCALE came up. In the web UI the “storage” pool was visible. I was able to import it. After importing the storage dataset and all its children were visible in the WebUI. It shows “Unencrypted” in the “Encryption” column. For good measure I tried the cli and the whole pool was mounted at /mnt/storage, and I was able to cat a random text file that was on the drive.
This seems a pretty conclusive demonstration that none of the drives in the pool is encrypted, and I will be able to migrate to SCALE. The only thing I can think of would be if migration has some weird edge case that fails for whatever reason is causing the CORE web UI to show it as “Legacy Encrypted” when it seems impossible from the rest of the evidence.