A descending index is real in 8.0 and matters for one shape

Earlier versions parsed DESC in an index definition and ignored it, which is worse than rejecting it — the index looked right and the query did a filesort.

CREATE INDEX idx_feed ON posts (author_id ASC, published_at DESC);

-- served without a sort in 8.0, filesorted in 5.7
SELECT * FROM posts
WHERE author_id = 4 ORDER BY published_at DESC LIMIT 20;

-- and the check
EXPLAIN ... -- Extra should NOT say 'Using filesort'

It only matters for a mixed-direction sort; a single-column index has always been readable backwards at no cost. The case it solves is exactly the feed query above — a leading equality and a descending sort on the next column — which before 8.0 meant a filesort or a stored negated column. Comparing the Extra column before and after is the only way to know it helped.