https://logstash-beta.wmflabs.org/app/kibana#/dashboard/mediawiki-deprecated should include the notices at T249982#6266618 after I manually attempted to trigger them, but instead it shows "No results found" - using the 6 month view only turns up a single entry from February
This is one of many dashboards that don't include any results, but arguable one of the more important, since it would prevent deprecation alerts from reaching production
The logstash instance in beta has an outdated version of the dashboard. I've deleted it as such.
For a while now, these are now emitted natively through PHP and no longer in a separate (largely ignored) channel. Use mediawiki-errors dashboard intead. Or, create a copy of it where there is a message:deprecated filter (like the production dashboard).
Note that the Logstash index for MediaWiki on Beta Cluster is currently empty due to T233134.