Deployment history has been dropped from the plan limits. The free plan listed a cap of 10 deployments per environment. That's gone, nothing prunes your history on any plan, and every plan shows the 50 most recent deployments for an environment.
Deployment history is the audit trail for an environment, including what changed, who deployed it, what to roll back to. Getting value from environment history requires history, so we decided that's not something we want to limit.