Celestia officially releases OP Stack x Celestia as Ethereum fallback solution, tldr:

1. Ethereum fallback is a mechanism that allows Ethereum Layer 2/Layer 3 to reuse Ethereum call data and use data availability when Celestia is down, which can prevent funds from being trapped on the Layer 2 bridge.

2. Currently Celestia supports this feature on OP Stack and Arbitrum Nitro, which is an optional feature.

3. When an error occurs when the Layer2 sorter uses PayForBlobs transactions, it will fall back to Ethereum and use Ethereum DA. After the interruption is resolved, it will resume sending data to Celestia. https://docs.celestia.org/developers/ethereum-fallback