Anyone know of any printed effects that key off of # of HD?

[PhoenixPhyre]

Where effects (if any!) are things like "making someone a valid target for an ability/spell". Basically any ability, spell, trap, item, anything, which directly references or does different things directly based on the total number of HD the target or creator has.

Things that don't count: anything that keys off of maximum or current HP. Anything that keys off of CR or level.

Things I already know about: recovery of spent HD during a long rest.

[JNAProductions]

Imprisonment keys the price of the gem off HD I believe.

And… that’s it, to my knowledge.

[PhoenixPhyre]

Imprisonment keys the price of the gem off HD I believe.

And… that’s it, to my knowledge.

It does. And that makes it icky to use as a player, because (and the reason I asked the question) CR is only very loosely coupled to HD (all else equal, more HD means possibly higher CR, but you have to change HD by quite a lot[1] to change CR by 1 even ceteris paribus, and ceteris is rarely paribus). So as a player who doesn't memorize the monster manual, how expensive of a component are you supposed to get? I guess you could go big and get a 33x500 = 16,500 gp item which should do you for most things (the tarrasque has 33 HD), but that's a bit overkill.

[1] To go up one CR step with nothing else changed, you have to go up by 2 defensive CR steps. Each defensive CR step is 15 HP wide, except at CR 21+, where they're 30 HP wide. So for CR <= 20, you need to go up 30 HP to raise by 1 CR. For a medium creature, each HD contributes 4.5 + CON HP. At 20 CON, that's slightly over 3 HD per CR. But if you have different CON or are bigger or smaller...yeah. It could be as little as 1.5 HD (gargantuan, 30 CON) or as much as 60 (tiny creature, 6 or 7 CON, for 0.5 HP/HD gained). Any lower on CON means you don't gain HP on level up or can actually lose them as a tiny creature.

[Kane0]

Healer feat.

[PhoenixPhyre]

Healer feat.

Good one. There, the only difference between total HD and level is that you can heal non-PCs with it (who don't reliably have levels).

So I guess that monster HD aren't only good for
* rolling HP if you don't want the average[1]
* the case where a monster stat block ends up taking short rests to heal using HD and we have to care about that. Maybe relevant for mounts and pets? Wild-shaped druids?

[1] I have a strong suspicion (and have always done myself) that HD are back-calculated from desired average HP, not the reverse. Ie, they say "I want about 150 HP, he's got a Con of 3 and is medium, so that's...<calculator>...20d8 + 60", rather than "I want 20 HD and a Con of +3, so that's...<calculator>...150 average HP."

[GeoffWatson]

Shepherd druid summon bonus hp.

[JackPhoenix]

I have a strong suspicion (and have always done myself) that HD are back-calculated from desired average HP, not the reverse. Ie, they say "I want about 150 HP, he's got a Con of 3 and is medium, so that's...<calculator>...20d8 + 60", rather than "I want 20 HD and a Con of +3, so that's...<calculator>...150 average HP."

Well, monster creation rules in the DMG outright tell you to pick the HP for the desired CR from the table multiple times, and HD is only mentioned as alternate method, being pretty much "if you really want to, pick whatever, it doesn't matter, and calculate HP from that", so I guess your suspicion may be correct.

[Amnestic]

Shepherd druid summon bonus hp.

This one is quite relevant for those using Tasha's summon spells since they don't have HD listed - just flat HP amounts, increasing with spell slot level.
Notably Fizban's dragon spirit does have HD listed though (HD=spell slot level).