When triaging errors for a new deployment it helps to filter out previous releases while in the earlier phases of a progressive rollout. This way you start empty, and can triage things as they come in (either filtering them out, or reporting them on phab etc.).
The issue is that right now this means fatal errors are omitted as mwversion:<something> will filter out events both from the old and new deployments as they don't carry this field.
This is technically not something we can know from the post-shutdown stage in the php-wmerrors handler, but I think we can do a best-effort approach here by plucking it out of the entry point's file path and fallback to doing what we do today (which is to not report it).
Perhaps in a future for multiple php-fpm instances and/or containers, this would be trivial if e.g. the deployed version is identified through an environment variable, but we're not there (yet).
The php-wmerors handler is maintained through puppet.git.