The same author also wrote an image display tool called Xee. 2.2 would run on 10.4 PowerPC. After Unarchiver's purchase, it seems Xee was part of the same deal and now only Xee 3 is available.
Fortunately my inveterate digital hoarding habit came in handy, because I managed to get both a working archive of The Unarchiver 3.11.1 and Xee 2.2 and the source code, so I can try to maintain them for our older platforms. (Xee I have compiling and running happily; Unarchiver will need a little work, but it's doable.) But that's kind of the trick, isn't it? If I hadn't thought to grab these and their source code a number of months ago as part of my standard operating procedure, they'd be gone, probably forever. I'm sure MacPaw (the new owners) are good people but I don't foresee them putting any time in to toss a bone to legacy Power Macs, let alone actually continue support. When these things happen without warning to a long-time open source free utility, that's even worse.
That said, the X Lossless Decoder, which I use regularly to rip CDs and change audio formats and did donate to, is still trucking along. Here's a real Universal app: it runs on any system from 10.4 to 10.12, on PowerPC or Intel, and the latest version of July 29, 2017 actually fixes a Tiger PowerPC bug. I'm getting worried about its future on our old machines, though: it's a 32-bit Intel app, and Apple has ominously said High Sierra "will be the last macOS release to support 32-bit apps without compromise." They haven't said what they mean by that, but my guess is that 10.14 might be the first release where Intel 32-bit Carbon apps either no longer run or have certain features disabled, and it's very possible 10.15 might not run any 32-bit applications (Carbon or Cocoa) at all. It might be possible to build 64-bit Intel and still lipo it with a 32-bit PowerPC executable, but these are the kinds of situations that get previously working configurations tossed in the eff-it bucket, especially if the code bases for each side of the fat binary end up diverging substantially. I guess I'd better grab a source snapshot too just in case.
As these long lived apps founder and obsolesce, if you want to something kept right, you keep it yourself.