SUMMARY
iBlockList apparently considers KIO's default user agent to be "impersonat[ing] browsers when accessing our service so [they] have put a stop to that."

In practice, "Mozilla/5.0 (X11; Linux x86_64) KIO/5.88 ktorrent/20.12.3" is being sent, so I guess they consider "Mozilla" to imply a web browser.

STEPS TO REPRODUCE
1. Enable IP filter plugin
2. Attempt to update it from the default URI

OBSERVED RESULT
Error

EXPECTED RESULT
Successful IP list download

ADDITIONAL INFORMATION
I was able to downgrade KIO to 5.80 to get the Browser Identification module for System Settings, then used that to spoof a Wget user agent for list.iblocklist.com. After upgrading KIO back to 5.88, KTorrent now works again.

It seems like perhaps the Browser Identification module should be restored, and KTorrent's default user agent adapted to not imply a browser (if iblocklist is stubborn and keeps the block as-is).