As FK errors, like deadlock errors are user errors, something like SQL_ERROR_LOG plugin, which is a simple AUDIT_PLUGIN that just counts, and then expose counts as either status variables or a information_schema table like locales .

Yes, this extends beyond FK, but just anticipating the next feature request

Or to allow the SQL_ERROR_LOG plugin to write into a table, which allows easy aggregation

Right, with the SQL service inside the server now that should be achievable.

The initial idea/requirement is/was a status counter for monitoring (and graphing)...
SHOW GLOBAL STATUS LIKE 'innodb_foreign_key_errors';