The Ethereum (ETH) Muir Glacier hard fork deployed at block 9.2 million, which occurred in the early morning hours on January 2. The fork appears to be quite successful, with 93% of the network running Muir Glacier, and the number of nodes which have not yet upgraded is rapidly decreasing.
The Muir Glacier hard fork delays the difficulty bomb by 4 million blocks, meaning the difficulty bomb will not start to kick in again until over 600 days from now.
The reason this fork is happening only a month or so after the Istanbul hard fork is that the difficulty bomb has become unpredictable. It was supposed to start kicking in around the middle of 2020, but instead began going off even before the Istanbul hard fork happened. Block times rose from 13 seconds in October to 17 seconds right before Muir Glacier, and eventually the network would have slowed to a crawl.
Fortunately, the block time is now returning to 13 seconds, where it will remain throughout 2020 and most of 2021, bringing a period of stability for Ethereum (ETH) miners, users, traders, and investors.
More Ethereum (ETH) forks are expected in the future as Ethereum (ETH) continues to transition towards Proof of Stake (PoS), but with the difficulty bomb out of the picture for awhile the need for forks will be less dire.