I've done the update and it has bricked my KT. Here is a description which is part of the email I sent to Kobo in the unlikely event that they actually respond with something useful.
I updated my Kobo Desktop for Mac to the new version 3.0.0 (actually, I wasn't given a choice not to!) and the new Desktop required me to update my Kobo Touch firmware to the new version 2.0.0 from 1.9.16. Everything appears to work until after I am told to unplug the Touch and the Touch reboots. The Updating screen appears and the hourglass flows for a while then the unit starts going through its startup routine and the image of the Touch with a smily face is shown with 5 squares below it. The squares sequentially go black and enlarge and shrink as you would expect. After a few passes on the squares, the unit stops and the 4th square is in its expanded position but nothing further happens. I left it for an hour thinking it might just need more time to finish the process but it is still showing the same thing and will not complete the startup process.
I have tried doing a soft reset
This causes the Touch to go through the startup routine again and sure enough, it stops in the same place.
Turning it off
By holding the off slider for a while, it seems to cause the Touch to restart which just like the soft reset, goes through the start up routine and stops in the same place.
Performing a Factory reset
I can't perform a factory reset in the normal fashion because the Touch will not start up. However, holding the home key down and performing a soft reset (keeping both held down for 10 seconds) causes a factory reset. I plug the Touch back into the computer and Kobo Desktop again updates the Touch. However, that too follows the same sequence and the Touch refuses to proceed past the same point in the startup routine.
Uninstalling Kobo Desktop and installing an earlier version
I tried uninstalling an earlier version of the Desktop. Unfortunately, once installed, it requires me to update to Version 3.0.0 and Desktop 3.0.0 subsequently forces me to update the Touch to 2.0.0 with the same results as above.
So, I see there is discussion about people using earlier firmware versions but it seems now, the Desktop forces you to update. Does anyone have any suggestion how I might get around this?