There were the canary contracts, which while they were active (they've been deactivated in mid 2016).
These simply stated whether a chain was good or bad, and they were controlled by the makers of ethereum. I don't know if they were ever used to mark a chain as bad, but they certainly represented centralized control.
I do see your point that most of the time, ethereum functioned using the decentralized protocol, but there was a centralized fallback.
From what I gathered from the links below, they were intended to stop mining if an unintentional hard fork would have happened. Or maybe also to force miners to upgrade? Did the Frontier release not yet have the ice age mechanism?
The wiki page suggests that there was an option to ignore the canary contracts. So you could mine nevertheless without recompiling the source.
There's no such option with IOTA as the coordinator is closed source.
https://github.com/ethereum/wiki/wiki/Bad-Chain-Canary
https://blog.ethereum.org/2015/07/22/frontier-is-coming-what...
Ethereum never depended on a centralized master node.