MariaDB 10.8

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select version();

version()

10.8.8-MariaDB-1:10.8.8+maria~deb11

-- 10.8 made descending indexes functional; before it, DESC in an index
-- definition was accepted and ignored. A fiddle cannot show the difference,
-- so this is only the syntax:
create table di (x int, index ix (x desc));
insert into di values (1),(2),(3);

Records: 3  Duplicates: 0  Warnings: 0

select x from di order by x desc;

x

3

2

1