The optimization process and recommendations:

1. Avoid OR Conditions By Using UNION (modified query below): In mosts cases, filtering using the OR operator cannot be applied using indexes. A more optimized alternative will be to split the query to two parts combined with a UNION clause, while each query holds one part of the original OR condition.
2. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
3. Use UNION ALL instead of UNION (query line: 44): Always use UNION ALL unless you need to eliminate duplicate records. By using UNION ALL, you'll avoid the expensive distinct operation the database applies when using a UNION clause.

Optimal indexes for this query:

ALTER TABLE `a` ADD INDEX `a_idx_type_deleted` (`type`,`deleted`);
ALTER TABLE `ass` ADD INDEX `ass_idx_borrower_id_created` (`borrower_id`,`created`);
ALTER TABLE `b` ADD INDEX `b_idx_id` (`id`);
ALTER TABLE `n` ADD INDEX `n_idx_deleted_status_lenderu_created` (`deleted`,`status`,`lenderUserId`,`created`);
ALTER TABLE `n` ADD INDEX `n_idx_lenderuserid_created` (`lenderUserId`,`created`);

The optimized query:

SELECT
        borrower_id,
        a_created,
        initiator_team_id,
        initiator_user_id,
        old_team_id,
        old_user_id,
        new_team_id,
        new_user_id 
    FROM
        ((SELECT
            a.borrowerId AS borrower_id,
            a.created AS a_created,
            NULL AS initiator_team_id,
            NULL AS initiator_user_id,
            NULL AS old_team_id,
            NULL AS old_user_id,
            n.teamId AS new_team_id,
            a.lenderUserId AS new_user_id 
        FROM
            a 
        INNER JOIN
            b 
                ON a.borrowerId = b.id 
        INNER JOIN
            n 
                ON a.lenderUserId = n.lenderUserId 
                AND a.created >= n.created 
                AND (
                    (
                        n.deleted IS NULL 
                        AND n.status = 'active'
                    )
                ) 
        LEFT OUTER JOIN
            ass 
                ON a.borrowerId = ass.borrower_id 
                AND a.created = ass.created 
        WHERE
            a.`type` = 'created' 
            AND a.deleted IS NULL 
            AND ass.borrower_id IS NULL) 
    UNION
    DISTINCT (SELECT
        a.borrowerId AS borrower_id,
        a.created AS a_created,
        NULL AS initiator_team_id,
        NULL AS initiator_user_id,
        NULL AS old_team_id,
        NULL AS old_user_id,
        n.teamId AS new_team_id,
        a.lenderUserId AS new_user_id 
    FROM
        a 
    INNER JOIN
        b 
            ON a.borrowerId = b.id 
    INNER JOIN
        n 
            ON a.lenderUserId = n.lenderUserId 
            AND a.created >= n.created 
            AND (a.created < n.deleted) 
    LEFT OUTER JOIN
        ass 
            ON a.borrowerId = ass.borrower_id 
            AND a.created = ass.created 
    WHERE
        a.`type` = 'created' 
        AND a.deleted IS NULL 
        AND ass.borrower_id IS NULL)
) AS union1