I don't have OS 9 installed on my puter, but I found this in the manual on the CD:
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Requirements
DiskWarrior supports any Mac OS Standard (HFS) or Mac OS Extended (HFS Plus) disk
including RAIDs, journaled disks, case-sensitive disks, FileVaults and iPods. DiskWarrior requires a G3 Mac or better (such as a G4, G5 or Intel Core-Duo) with built-in FireWire and a minimum 256 MB RAM (512 MB recommended) to startup and run from the DiskWarrior disc.
To install, your must be started from Mac OS X v. 10.3.9 or later.
The disk to be rebuilt must be locally connected, must not be write protected, and must have a valid partition map. If you plan to rebuild the directory on your startup disk, you need to start up from another disk capable of starting up Mac OS X v. 10.3.9 or later, such as the DiskWarrior Disc (refer to Rebuilding Your HFS and HFS Plus Disks on page 23 in this chapter).
I would surmise that most people running OS 10.3.9 or higher haven't got separate installs of OS 9 and Alsoft figures that those needing DW 2.1 AND DW 4 who don't already own DW 2.1 or 3.x are few and far between. After all, OS X will be 6 years old this coming year (non-beta), as will Diskwarrior 2.1. In computer years, that's a long time. At Alsoft's web site in the section on system requirements for DW 4, it says the following:
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If your Mac is running Mac OS 9.2.2 or earlier, DiskWarrior 2.1 is still available for purchase directly from Alsoft. Please contact our Sales Department to order.