MySQL 8 already allows this:
Using GROUPING() to substitute labels for the super-aggregate NULL values makes the result easier to interpret:
IF(GROUPING(name) = 1, 'All items', name) AS name,
IF(GROUPING(size) = 1, 'All sizes', size) AS size,
SUM(quantity) AS quantity
GROUP BY name, size WITH ROLLUP;
This is an old feature of MsSQL. Over the years I have filed a few times a request so MariaDB fixes this problem because we cannot return nulls in the grouping cells, we need to add a meaningful label. Please stop procrastinating and listen to the business people. We cannot work efficiently as it is.