I suddenly could not get to all but a few devices on my local network. Attending each missing one, they all had non-routable IP addresses "assigned" from DHCP.
Available devices resided on a different switch, so I looked at the offending switch activity. Its LEDs all blinked heavily in a synchronous pattern.
At first, I suspected the switch itself after disconnecting almost every eth cable from each switch port (turns out I should have waited longer).
I brought in an ancient router I still had which had enough ports on it to temporarily replace the offending switch, but that did not do it.
By reconnecting each eth back to the offending switch, waiting for connection and traffic LEDs, then adding another, the offending device caused that synchronous pattern.
Disconnected that device then continued adding the rest with normal asychronous LED patterns.
The other devices even got their normal DHCP leases and started working normal.
All in all, about an hour's worth of disruption.