MariaDB 10.5

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CREATE TABLE mytable (
  myjson JSON
);

INSERT INTO mytable VALUES
('{"arrayOfDates" : ["2020-01-11", "2021-01-12", "2019-03-12"], "key1": "value1"}')
select REPLACE(REPLACE(JSON_QUERY(myjson, '$.arrayOfDates'), '[', ''), ']', ',')
from mytable


REPLACE(REPLACE(JSON_QUERY(myjson, '$.arrayOfDates'), '[', ''), ']', ',')


"2020-01-11", "2021-01-12", "2019-03-12",


with recursive cte as (
      select '            ' as dt, REPLACE(REPLACE(JSON_QUERY(myjson, '$.arrayOfDates'), '[', ''), ']', ',') as dates, 1 as lev
      from mytable
      union all
      select substring_index(dates, ',', 1),
             substr(dates, instr(dates, ',') + 2), lev + 1
      from cte
      where dates like '%,%'
)
select MAX(dt) AS max_date
from cte
where lev > 1


max_date


"2021-01-12"