Document default change for legacy-replication-lag-algorithm flag

Author: promptless[bot]

content/en/docs/24.0/reference/programs/vtgate/_index.md

@@ -126,7 +126,7 @@ vtgate \
       --keep-logs-by-mtime duration                                      keep logs for this long (using mtime) (zero to keep forever)
       --keyspaces-to-watch strings                                       Specifies which keyspaces this vtgate should have access to while routing queries or accessing the vschema.
       --lameduck-period duration                                         keep running at least this long after SIGTERM before stopping (default 50ms)
-      --legacy-replication-lag-algorithm                                 Use the legacy algorithm when selecting vttablets for serving. (default true)
+      --legacy-replication-lag-algorithm                                 (DEPRECATED) Use the legacy algorithm when selecting vttablets for serving.
       --lock-heartbeat-time duration                                     If there is lock function used. This will keep the lock connection active by using this heartbeat (default 5s)
       --lock-timeout duration                                            Maximum time to wait when attempting to acquire a lock from the topo server (default 45s)
       --log-err-stacks                                                   log stack traces for errors

content/en/docs/24.0/user-guides/configuration-advanced/query-load-balancing.md

@@ -5,6 +5,14 @@ weight: 12
 
 For applications which scale out read queries using replicas, Vitess can safely avoid sending queries to replicas with replication lag beyond acceptable thresholds.
 
+{{< info >}}
+**Note:** Starting in Vitess v24, VTGate uses a simpler default algorithm for handling replication lag based on low lag thresholds, high lag thresholds, and minimum number of tablets. This algorithm is more stable in production environments.
+
+The legacy algorithm is deprecated and will be removed in a future release.
+
+For more details about the algorithm change, see the [v24.0 release notes](https://github.com/vitessio/vitess/blob/main/changelog/24.0/24.0.0/summary.md#vtgate-new-default-legacy-replication-lag-algorithm).
+{{< /info >}}
+
 ## Flags
 
 You can set the following flags on VTGate to control load-balancing of read queries for replicas: