The ddlLockOptimisticRecoveryWaitTimeout is a cluster parameter aimed at supporting the extremely unlikely scenario in which a user would like to override the default 100ms timeout for ddl lock recovery that was set under SERVER-111794.

The parameter has always intended to be internal but apparently some users have reported having persisted it, leading to downgrade failures (the parameter was added in v8.0). This ticket is either to properly guardrail it (via the min_fcv cluster parameter option) to avoid downgrade failures or to directly remove it, given its doubtful usefulness.