A knife fight on horseback was called a Cavalry Engagement - fast-moving, bloody affairs...
The common soldier - by mid-war - would have had almost zero use for something like a boot knife or dirk, unless he'd figured out something else he could do with it.
By that time, an experienced soldier on campaign was stripped down to the barest of essentials - usually a rubber blanket, canteen, spoon and cup, and cartridge pouch.
The spare shirt and socks (if any) were rolled up inside the blanket - the issued haversack proving an encumberance.
A 'boot knife' is one of those things that seems like a good idea, until it's tried and it's found to be more of a pain (literally) than it's continued use would warrant.
The most famous users of the concept were American Paratroopers of WWII - who strapped an M3 Trench Knife to the sides of their Corcorans, in the event they'd get hung up in a tree or otherwise rendered weaponless.
Those M3's would find their way back onto the webbing as soon as possible, because the damned things caught on everything one can find on the ground.
Vaya,
Scouts Out!