MediaWiki core currently has a compose dependency on "wikimedia/avro". It's used in AvroFormatter for monolog, that AFAIK was used for analytics events logging for action API requests and for CirrusSearch requests. Both features has been successfully migrated to eventgate. Additionally, we have standardized on using JSON instead of Avro.
I propose to drop Avro monolog features and remove the dependency. It's only suggested and required as a dev dependency for testing. It is very highly unlikely anybody else was using these, and I'm not sure these will even require deprecation.