Monster types are a stupid rule

I have said it before and I say it again: the monster type mechanic in D&D is stupid. The monster type mechanic is both poorly defined, nonsensically rigid, biologically insane and makes no sense from a holistic world building perspective.

Ironically this was not always the case: the concept of monster types was introduced in the Rules Cyclopedia and the types had almost no rules attached and multiple types could be applied to a single monster. Wizards of the Coast ruined everything by forcing the types to become the equivalents of class levels and attached all sorts of baggage like the "type hierarchy" that requires absurd two-axis tables to determine the type of a "hybrid" rather than have both.

3.0's unrefined monster type mechanic is a truly hilarious read. The types include "beast" and "shapeshifter", among others. Non-humanoid animals are classified as either animals (real animals), beasts ("ahistorical" animals), magical beasts (intelligent or "magical" animals) or vermin (mindless invertebrates). Crazy, right?

Many d20 derivatives have attempted to pair down the monster types, but only 4e, FantasyCraft and 13th Age did a remotely good job. Pathfinder and 5e just repeat the mistakes of the past. While 5e reduces types back down to simple tags, it idiotically refuses to let any monster have multiple types and the unbalanced rules for summoning and polymorphing rely on the type mechanic (e.g. spells that summon fey are screwed because there are few fey in the monster books).

Some 3.x supplements actually added new types to address gaps in the already flawed mechanic. Infernum added the "biomechanoid" type, Relics & Rituals: Excalibur added the "manifestation" type, Encyclopaedia Divine: Druids added the "spirit" type. Naturally, they had much clearer definitions than some of the core types did.

Some supplements tried to give clear definitions to the types, which had no success since nobody else adopted the idea. Legends & Lairs: Darkness & Dread defined "abominations" (aberrations) as things from beyond cloaked by the universe in hunks of flesh. The Complete Guide to Fey defined fey as spirits trapped in material forms. A Magical Society: Beast Builder gave up and outright stated that typing an aberration was completely arbitrary.

The Colours of Magic series by Plot Device once attempted to justify the 3.x types by tying them into different colors of magic. This proved messy and only highlighted the flaws of the type mechanic.

One 3.x house rule I read tried to divide types into super types, body types, mind types, and soul types. These determined how the monster interacted with other rules, such as whether it was alive, mindless, soulless or whatever. I thought the idea was good but the execution was marred by the baggage of monster levels.

Monvesia goes in a whole other direction...

OSR games generally do not have formal type systems, and if they do then there is no formal hierarchy. A bugbear can be both a beastman and a goblinoid, for example. Mazes & Minotaurs has only four types: animates, beasts, folks, monsters and spirits. That is enough to classify pretty much everything.

That reminds me... why are undead traditionally ruled as immune to mind-affecting effects? Magic? That has never made sense to me.