Upgrade streaming replication and log-shipping standby servers

Dear Colleagues,

When upgrading to a new major version, pg_upgrade documentation
https://www.postgresql.org/docs/current/pgupgrade.html#PGUPGRADE-STEP-REPLICAS
states that pg_upgrade can be used on a master server. After upgrading
the PostgreSQL version on standbys, however, one must either pull the
data from scratch with pg_basebackup, or run rsync from master to
standby.

Is running pg_upgrade on replicas not supported and why?

-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
2:5005/49@fidonet http://vas.tomsk.ru/