something that has ceased to be useful or relevant.
Before X Was X: The Dark Horse Story Of The 24th LetterSince its inception, the letter X has struggled to establish its own identity, so it may be no coincidence that X is commonly used to represent the unknown in both language and mathematics. But, how was it created?
beat/flog a dead horse, to persist in pursuing or trying to revive interest in a project or subject that has lost its usefulness or relevance.
Origin of dead horse
An Americanism dating back to 1820–30
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for dead horse
So side by side and hand in hand the two of us walked out into Dead-Horse Lake.The Prairie Child|Arthur Stringer
Idioms and Phrases with dead horse
see beat a dead horse.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.