We met with @Ottomata who brought to light the work on client side error logging. It was indicated that this task should be filed to serve as notice about upcoming changes to logstash that potentially affect this work.
As part of T234565, logstash will adopt Elastic Common Schema as the schema for log events. Client error logging should attempt to use ECS-defined fields when possible to stem the probability of dropped fields and ease the migration of the stream to the new schema once ratified.
Unfortunately, ECS cannot be adopted fully until the legacy logstash cluster is decommissioned due to mapping conflicts with the current mapping configuration. Looking at jsonschema/mediawiki/client/error/1.2.0.yaml, it looks like only the url field is affected.