What is Ethereum's Arrow Glacier hard fork?

Arrow Glacier hard fork

Mainnet 2021-12-09

The difficulty bomb was delayed.

Users: Nothing changes directly. Arrow Glacier contained only one change, which pushed back the difficulty bomb. The difficulty bomb is a mechanism that, from a set point in time, sharply raises the proof-of-work mining difficulty and slows block production. Because it was delayed before the bomb took effect, block intervals, fees and the way wallets are used stayed the same as before. You could keep using your existing wallets and apps as they were.
Wallet & app developers: Nothing changed. There was no new opcode, transaction format or gas cost change, so there was no need to modify smart contracts or app code. Teams that use an external RPC provider only had to confirm that the provider had upgraded its clients. Teams that run their own nodes needed the same upgrade as node operators. Because contract behavior was the same before and after the fork, no separate user announcement was needed either.
Node operators: These were the most directly affected. A hard fork changes the rules at a set block height, so before then clients had to be upgraded to a version that supports Arrow Glacier. Nodes that did not upgrade stayed on a chain following the old rules after the fork and split off from mainnet. Miners also had to upgrade their clients, and as a result they did not experience block times lengthening because of the difficulty bomb. EIP-4345 pushed the bomb back to around June 2022, buying time to prepare for the Merge.