The three status fields people read all measure something slightly different from what the question was, and knowing which is which is the difference between a useful alert and a false one.
SHOW SLAVE STATUSG
-- Slave_IO_Running: is it RECEIVING events?
-- Slave_SQL_Running: is it APPLYING them?
-- Seconds_Behind_Master: how far behind the events it HAS.
--
-- all three can be healthy while the data is an hour old.
-- 8.0.22 renames these to REPLICA; both spellings work for now.Alerting on all three plus the heartbeat is the arrangement that covers the cases: the IO thread stopping is a network or credential problem, the SQL thread stopping is usually a duplicate key, and a rising heartbeat with both threads healthy is a slow statement. A replica that is up, accepting connections and serving hour-old data is worse than one that is down, because nothing routes away from it automatically.