Get the value of the first item of JSON object
OK so the final query is :

WITH status_per_table AS (
SELECT
  changes,
  stmt_date :: DATE
FROM pgmemento.table_event_log el
  JOIN pgmemento.row_log r ON (r.event_id = el.id)
  JOIN pgmemento.transaction_log t ON (el.transaction_id = t.txid)
WHERE r.changes ?| (SELECT array((SELECT DISTINCT key
                                  FROM pgmemento.row_log, LATERAL jsonb_each_text(changes)
                                  WHERE key LIKE '%_stat'
                                  GROUP BY key)))
) SELECT
    value,
    stmt_date,
    count(value)
  FROM status_per_table, LATERAL jsonb_each_text(changes)
  GROUP BY value, stmt_date
  ORDER BY stmt_date DESC


Don't hesitate to post a  suggestion from a new post name...