[upgrade to 7.x] xpack.notification.slack.account.<account_name>.url settings get archived without notice

[chingis-elastic]

It seems that starting from 6.6 watcher settings for Slack account URL

xpack.notification.slack.account.<account_name>.url

are deprecated and get silently converted into archived.* after upgrade to 7.x. This could leave cluster in broken state after the upgrade as described in https://github.com/elastic/elasticsearch/issues/28026. It would be nice to have deprecation/upgrade assistant warning for user to take action.

To ensure smooth transition to 7.x from versions prior to 6.6, workaround is to upgrade to latest 6.8, set corresponding settings via

xpack.notification.slack.account.<account_name>.secure_url

and remove old *.url settings.

Elasticsearch version (bin/elasticsearch --version): 6.8.10

Plugins installed: default at https://cloud.elastic.co

JVM version (java -version): default at https://cloud.elastic.co

OS version (uname -a if on a Unix-like system): default at https://cloud.elastic.co

Steps to reproduce: Same as in https://github.com/elastic/elasticsearch/issues/28026 with upgrade done from 6 to 7. An offending setting on 6.x cluster could be set as

PUT /_cluster/settings
{
  "persistent": {
    "xpack.notification.slack.account.metering.url": "test1"
  }
}

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Watcher)