Alsoft has released a "sort of" 10.13.x compatible version of Disk Warrior. It is a free download to current registered DW v. 5.0 users. Read all about it here.

I tried, twice, to download it and got a message that the .dmg file could not be opened. Suspiciously, the download occurred instantly. I emailed Alsoft tech support about the issue.

EDIT: Alsoft suggested that I restart the computer and try again. Problem solved!

The DW Project Manager posted this at MacUpdate :

"Apple made internal changes to High Sierra (macOS 10.13.x) that required we at Alsoft make some minor changes to DiskWarrior and release those changes to our customers. This is version 5.1. APFS work is ongoing, but we cannot safely release a full APFS rebuilding feature without details we need from Apple, that we have not received. In the new DiskWarrior 5.1, APFS disks are recognized, but rebuilding is not available. Additionally, there is a new version of DiskWarrior Recovery Maker, version 1.2, which is included with DiskWarrior 5.1. DiskWarrior Recovery Maker 1.2 adds support for updating and creating a startup flash drive from APFS-based startup drives. Thank you to our customers for your support. We will publish a version of DiskWarrior with APFS support as soon as we are able, but this is not entirely up to us."

If I understand this correctly, DW v5.1 is indeed compatible with APFS, but it still can't repair the directory.

FWIW, I installed v5.1 on 3 different HFS+ machines and tested it on all volumes. As expected, no issues ensued (well, at least not yet).

Moi aussi! I installed it on my one and only computer and it works without issue.

Exactly correct, hence my "sort of" comment.