I just went through a DEF derate about a month ago. I live in far northeast, remote and very rural California. While traveling home from a trip the "Service DEF System, See Dealer" warning came up and gave me 91 miles before derate. My closest dealership, which is my dealer, was 110 miles away, while home was 96 miles away. I chose home, and when it it hit that 91 miles derate threshold another warning came up that told me the truck would go into actual 5mph limp mode at either the next engine shutdown, refueling, or extended idle time (what that time is I do not know).
The truck operated normally and I got it home without a derate. I had it towed to a local mechanic who got me back on the road. I haven't taken it to a dealership yet (my dealership's very good mechanics both jumped ship and went to work for the local municipality), but I'm hoping it was simply a glitchy sensor. Something like this proposal would give us some breathing room to get our trucks safely to the right place without the hassle and expense of towing.
Gawd, the EPA is an awful agency staffed with truly awful people. I hope Admin Zeldin can make it a more sensible PUBLIC SERVICE agency.