Is The trope that wizards can't use armor just an arbitrary limitation?

[David Johnston2]

Originally Posted by Astromancer

There are armored spell casters in Arthurian lore, they are generally not represented in Mallory. Icelandic Wizards certainly wore armor and had reputations for being awesome warriors (heck Gandalf used a sword). The Elf king in the English Faerie tale Childe Rowland was a steel armor wearing fae warrior wizard. Warrior Wizards do show up in many traditions.

I'm not saying warrior-wizards never existed in any work of fiction or folktale. That's obviously not true, although the nonhumans aren't relevant. They're precisely the reason why D&D made an exception for Elven fighter/magic users. But I am saying that there are a lot of bodies of fiction and myth where the wizards leave the body armour to the grunts so the statement that the restriction has no basis in fiction or legend is simply false.