Summary:

Currently Bamboo doesn't collect audit logs for Bamboo build deletions, like who deleted the builds.
These can be tracked only in atlassian-bamboo-access.log by grepping with the below pattern under <Bamboo-Home>/logs folder:

 $grep -i deletePlanResults atlassian-bamboo-access.log* 

Suggestion:

Track audit logs for the build deletion under Bamboo plans in audit logs in DB and under build plan configuration > audit logs.