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Looks like they migrated (at least partially) to Scylla: "Discord Chooses Scylla as Its Core Storage Layer" (2020)

Yes. They started experimenting with Scylla earlier, and made the switch in 2020. Here's more on their logic — they like "opinionated systems":

Scylla is really impressive. The only complaint I've heard is about the shard-per-cpu approach, causing some issues when data has bad distribution.