I don't train Muay Thai, but I always thought the gold standard for shin conditioning was kicking something hard at the core with a bit of softness or "give" on the outside, like the bannana trees that the Thai used traditionally.
Of course, "old school" doesn't necessarily mean "good;" if it did, responsible trainers wouldn't eschew stupid and dangerous training methods popular in the good ol' days, such as training like a maniac on hot days while denying yourself water so as to condition your body to perform better without it, or "salt loading" before competitions. My Dad, who spent a year in Thailand when I was a toddler, used to tell me stories of the boxers from the local school wailing away full-force with their shins on a plain teakwood pole. I'm sure they had some bad-ass hard shins, but I'd not recommend the practice; I'd wager more than one of those guys also has a bad-ass limp, bone damage, problems from old blod clots, or other problems in the 35 or so years since then related to their training.