Where can I read more about this Monero checkpointing?

MoneroPulse: a loosely distributed checkpoint alert system. Normal nodes occasionally check and warn, but you can also force the right fork

The addition of MoneroPulse, a loosely distributed checkpoint alert system. MoneroPulse will allow us to add both block hash checkpoints and blob hash checkpoints (to defeat attacks like the Block 202612 attack). For ordinary nodes that are checked at least occasionally, the daemon will notify you in angry red letters when your local chain doesn't meet a checkpoint. If you run an unattended node (merchant systems, pools, etc.) you will want to turn on the "--enforce-dns-checkpointing" flag so that these checkpoints are enforced and not merely notified.

support for long / blob hash checkpointing on both the file and DNS side.
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Core: DNS checkpointing is now asynchronous, and doesn't prevent blocks from being received (in testing)

Core: file and DNS checkpoints now also support blob hash (longhash) checkpointing (in testing)