[ASA Vet]

a deer that takes most of a tight spread of 0 or 00 at 40 yds will be
more able to move under its own power than one that takes a slug at 40 yds.

Complete agreement here. 35 yards is about max for buckshot.
I've used both slugs & buckshot.
Some places I hunt you can't see further than 10 - 20 yards.
0 or 00 buckshot is ok. In more open areas I'd use slugs.
Most of my deer hunting though is done in rifle areas
of Michigan, so I use a Savage in .300 win mag loaded with 165 gr Nosler Ballistic tips,
under a simmons 44 mag scope. It's zeroed at 200 yards.

[lugsoul]

"35 yards is about max for buckshot."

I really think that depends on your choke. With an Extra Full, you can stretch that to about 45 yds. But it is just not gonna knock a deer down like a slug.

10-20 yds? That is some thick vegetation! I would agree, however, that a load of buckshot at 15-20 yds packs quite a punch - assuming you aren't losing pellets to trees and limbs.

I haven't hunted deer with a shotgun in at least 15 years, but that is primarily because I don't have to. The shotgun is pretty much restricted to waterfowl and clays.