This is an instance of https://github.com/lightningdevkit/rapid-gossip-sync-server. It can be used with its client counterpart, documented here: https://docs.rs/lightning-rapid-gossip-sync/latest/lightning_rapid_gossip_sync/

curl -o rapid_sync.lngossip https://rapidsync.lightningdevkit.org/snapshot/<last_sync_timestamp>

On first sync, <last_sync_timestamp> is 0.