Many thanks for your reply, @DavidTurner !
Does this mean we need to upgrade the cluster from v8.7.1 to v8.18.x (v8.18.2) before trying to set that index to read-only?
Does only Elasticsearch need to be upgraded or Kibana as well?
Finally, would steps above be enough or reindexing that index to v8 will still be required?
I'm asking as some of the answers on this forum suggest that restoring indices older than one major version require reindexing to the previous major version first.
For example: Problem with restoring old (6.8) indices on ES 8.6.1
Elasticsearch can only read indices one major version back. Elasticsearch 8.x can therefore only read indices created in Elasticsearch 7.0 or newer. You will need to restore your 6.x indices into a 7.x cluster and reindex before the new indices can be snapshotted and restored into Elasticsearch 8.x.
Also, the answer in Snapshot index compatibility suggests that simply restoring the old index into previous major version Elastic is not enough to enable restoring it to the Elastic with the latest major version.
Thanks