How to find unused columns in MariaDB tables
If your table has columns that are defined to have NULL values, run this:
SELECT COUNT(given_column) FROM mytable;

This gives you the numbers of rows in mytable whose given_column is not NULL. If the result comes back as zero(0), that column is not populated with anything. You could drop that column with ALTER TABLE mytable DROP COLUMN given_column;
If your table has columns define as DEFAULT &lt;some-value&gt;, run this
SELECT COUNT(DISTINCT given_column) DistinctCount FROM mytable;

If this comes back as 1, you could most likely drop this column as well.
If the DistinctCount &gt; 1 and you want to see that breakdown, run this
SELECT IFNULL(given_column,'Total Rows') ColumnsValues,COUNT(1) rowcount
FROM mytable GROUP BY given_column WITH ROLLUP;

These queries should give you a good idea which can be dropped.
CAVEAT : Please make sure your given_column is never referenced in your source code before dropping the column