A variable that was removed rather than deprecated

A configuration file carried forward since 2018, containing a variable the new version refuses to start with.

$ docker compose up db
  [ERROR] unknown variable
  'binlog_transaction_dependency_tracking=WRITESET'
  [ERROR] Aborting

the variable was deprecated in 8.0.35 and removed in
8.4 — the behaviour is now the default and the setting
is meaningless rather than wrong.

and the reason it did not appear in staging: staging's
configuration had been rewritten in 2022 and
production's had not.

A refusal to start is the correct severity for an unknown variable and it means the failure is at boot rather than in behaviour, which is the good case. The finding that mattered more was the divergence between the two configuration files — a staging environment that does not share production’s configuration is testing something else.