dufing upgrade 7.4.2 -> 7.9 I received an error
"failed shard on node [GnAJTEqpTeyawpDyUli4Aw]: failed to create index, failure IllegalArgumentException[unknown setting [index.hidden] please check that any required plugins are installed, or check the breaking changes documentation for removed settings]", shard has exceeded the maximum number of retries  on failed allocation attempts - manually call [/_cluster/reroute?retry_failed=true]
so I executed recommended command:
curl -X POST -u elastic:xxxxxxx "http://localhost:9200/_cluster/reroute?retry_failed=true"
but cluster become RED state!!!
curl -XGET -u elastic:xxxxxx "http://localhost:9200/_cat/health" 1598883387 14:16:27 corp-cz-cem red 21 17 1336 711 0 0 2 0 - 99.9%
I found this shards are unallocated
curl -s -XGET -u elastic:xxxxxx "http://localhost:9200/_cat/shards" |grep -v STARTED ilm-history-2-000001 0 p UNASSIGNED ilm-history-2-000001 0 r UNASSIGNED
please could you help?
my production cluster is down and I am looking for any solution