Old replication slots prevent vacuum from advancing relfrozenxid

Replication slots with large age(xmin) or age(catalog_xmin) prevent VACUUM from advancing relfrozenxid, potentially leading to transaction ID wraparound. Slots may exist for servers that no longer exist or have been down for extended periods.

Check pg_stat_replication for slots where age(xmin) or age(catalog_xmin) is large. Drop slots created for non-existent or long-down servers. If dropping slot for active server, that replica may need to be rebuilt.