MySQL 8.0

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WITH Table1 AS (
    SELECT 'II-10' AS ID, 'RTE-011' AS Unique_Key, '2018-04-07 15:18:25' AS Date, 'er-12-tt' AS MapId UNION ALL
    SELECT 'II-11', 'RTE-011', '2018-05-17 11:44:47', 'er-13-td' UNION ALL
    SELECT 'II-12', 'RTE-011', '2018-06-12 13:22:29', 'ee-13-rt' UNION ALL
    SELECT 'II-13', 'RTE-012', '2018-04-07 19:17:14', 'eg-15-st' UNION ALL
    SELECT 'II-14', 'RTE-012', '2018-04-07 21:22:37', 'hr-15-yt' UNION ALL
    SELECT 'II-15', 'RTE-013', '2018-07-07 20:27:38', 'tr-16-yt' UNION ALL
    SELECT 'II-15', 'RTE-013', '2018-08-07 14:18:33', 'tr-16-yt'
),
Table2 AS (
    SELECT 'er-12-tt' AS Id, 'Open' AS Status UNION ALL
    SELECT 'er-13-td', 'Closed' UNION ALL
    SELECT 'er-13-rt', 'Closed' UNION ALL
    SELECT 'eg-15-st', 'Closed' UNION ALL
    SELECT 'hr-15-yt', 'Pending' UNION ALL
    SELECT 'tr-16-yt', 'Open' UNION ALL
    SELECT 'tr-16-yt', 'Pending'
)

SELECT
    ID_t1, Unique_Key, Date, MapId, ID_t2, Status
FROM
(
    SELECT t1.ID AS ID_t1, t1.Unique_Key, t1.Date, t1.MapId, t2.Id AS ID_t2,
        t2.Status,
        ROW_NUMBER() OVER (PARTITION BY t1.Unique_Key ORDER BY t1.Date DESC) rn
    FROM Table1 t1
    INNER JOIN Table2 t2
        ON t1.MapId = t2.Id
    WHERE t2.Status = 'Closed'
) t
WHERE rn = 1;

ID_t1	Unique_Key	Date	MapId	ID_t2	Status
II-11	RTE-011	2018-05-17 11:44:47	er-13-td	er-13-td	Closed
II-13	RTE-012	2018-04-07 19:17:14	eg-15-st	eg-15-st	Closed