Here, I'm attaching a different external USB disk.
[109000.437697] usb 2-4: new SuperSpeed Gen 1 USB device number 5 using xhci_hcd
But, that's it. It just stops there. These drives work fine on other linux desktop systems. Here's what I get on kernel 5.15:
[115122.399827] usb 4-3: new SuperSpeed USB device number 4 using xhci_hcd
[115122.412592] usb 4-3: New USB device found, idVendor=0080, idProduct=a001, bcdDevice= 2.03
[115122.412594] usb 4-3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[115122.412596] usb 4-3: Product: External USB 3.0
[115122.412596] usb 4-3: Manufacturer: TOSHIBA
[115122.412597] usb 4-3: SerialNumber: <redacted>
[115122.414525] scsi host10: uas
[115122.414992] scsi 10:0:0:0: Direct-Access TO Exter nal USB 3.0 0203 PQ: 0 ANSI: 6
[115122.415827] sd 10:0:0:0: Attached scsi generic sg5 type 0
[115122.416376] sd 10:0:0:0: [sdf] 234441648 512-byte logical blocks: (120 GB/112 GiB)
[115122.416378] sd 10:0:0:0: [sdf] 4096-byte physical blocks
Notice the delay between the first line and the next four lines. What is this second process that is occurring on my desktop system and not happening in OpenWRT? It looks like it's some kind of identification. After that, ...
aha! UAS! I see uas mentioned. Let me try that, again...
I installed kmod-usb-storage-uas and it worked immediately!
So, I thought I had tried that package before, and it didn't help. In fact, I know I tried it, because I installed it and then removed it, and that is why it didn't show up in opkg list when I tried to find it again, yesterday. It wasn't in opkg list until I did opkg update again. As a result, I thought the documentation was outdated and that the -uas pkg didn't exist anymore on v21. I thought I remembered trying it on v19 and tricked myself into thinking I had seen it the package list recently. But, I didn't realize that removing the package would delete it from the list of available packages.
So, UAS was required for this particular drive, as was clearly stated in the "Using storage devices" guide. However, there was almost certaintly something else that was required to make it work, too; something in addition to what is described on that page. I have since installed so many extra packages, it is hard to be sure which one(s) were also required. I tried to check my buffer, but not enough history to even review.
Anyway, thanks again to the developers and the community here for keeping OpenWRT alive.