All clients that relied on the Hiro API. This includes the Explorer, Hiro desktop and web wallets, the stacks CLI, and others. Requests on all endpoints to the API were still being served, but were returning old data.
This incident was caused by the stacks-node broadcasting events to the writer API suddenly losing all outbound peers around 10:30AM ET. It wasn't noticed until a message was posted about it in Discord around 11:50AM ET. After being notified, traffic was switched to a different sub environment and the API was then able to return new block and transaction data.