This is actually a very interesting question. I assume that you are referencing extensions that add shards to Postgres, since it runs on a single server out of the box. You made me curious how those extensions/rewrites handle sequences.

It looks like Citus (one such extension that runs shards on separate workers) does something a little like what you described. Each node gets an ID, which becomes the high bits of the sequences it generates. So serial IDs are monotonic within a shard, but not across shards.

Aurora PostgreSQL Limitless uses different abstractions, but also ends up reserving different chunks of IDs for different routers to use.