The Chromebook Recovery Tool in my Acer c720 just stopped working. It now says your Chromebook must be at least 88.0 to use the tool.

http://www.geocities.ws/bkbrat/RecoveryTool.png

But I found where you can do a double Power-Wash and get ask if you want it to go back to the precious update.
So I backed it up again to the 74.0.3729.159 and so far it is staying there without going back to the 76.0.3809.136 update.