Ah, I found it. The Dark Wizard Of Donkerk by Alexander Wales, and the oathkeepers.
“To raise him,” replied Omarr. “If he went back to the orphanage he would more likely than not spend ten years eating gruel and getting whatever feeble education the Sisters provide him with. When he’s ten years old, he’ll be taken to the monastery and told to swear some oaths. If he keeps them, he’ll be an oathkeeper, bound to a pitiable existence in return for awe inspiring powers. If he doesn’t keep the oaths, he’ll be sent out into the world on his own with nothing but the clothes on his back and whatever he can steal.” Omarr had run away the day before initiation, but he’d seen the path that was being laid out in front of him.
....
One of the perks of being an oathkeeper was the fact that if anyone questioned whether you had broken your oath, you could simply make a twisting jump and land with perfect poise on the top of a two story building to answer them. The seven-pointed star on his chest signaled strongly to the people he met on the street, but not so strongly as the plainly visible power of his five oaths.
The magic armor is The Strangheid Armor.
Delland had kept drinking the wine, and even though it was likely watered, he’d become less coherent even since Ventor had entered the room. The Strangheid was one of a kind, a suit of full plate that kept the body strong without need for food or drink, but while it removed the need, it didn’t remove the desire. Though the wearer of the Strangheid would be as fit and healthy as if he were getting his daily meals, he always felt on the edge of starvation and dehydration. Delland had taken an Oath never to remove the armor, never to willingly imbibe even a drop of water, or snack on even a small morsel of food. He’d gone some sixty years without wine, and now he was trying to make up for the lost time all at once.
The Sisters are the distaff counterpart to the oathkeepers.
The forbidden magic is not precisely defined, but practitioners are referred to as "dark wizards, or black witches" and it's a form of pact magic, with the distinction basically being whether it involves human sacrifice.
“Pacts are a fine line,” said Hirrush. “If I sold a passing merchant one of our chickens, the two of us haven’t made a pact, we’ve made a transaction. If I told you that I was going to give you a cookie, that wouldn’t be a pact either, only a promise that I’ve made to you that I can freely break. For a true pact, you need some trust, and either that trust comes from neither party being able to break the pact, or from the pact holding firm for a long time. A pact needs consequences, or retribution, or something solid to make it real.”
....
“Not as such,” said Clarice. “Which is why I don’t credit it much. There are a handful of hedge witches within a day’s walk of here, but none that I know of that have done anything truly black. Animal sacrifice seems to be the worst of it. It’s not impossible that one of them has gone over the edge, I’ll grant that. And no dark wizards, to my knowledge.”
Other branches of magic include "the eloists, the elementalists, the denialists, the mentalists, the spirit callers" with the spirit callers being the lost form of magic I was thinking of.
“They were supposed to be like dark wizards that didn’t need to sacrifice anything, or oathkeepers that didn’t need to keep any oaths. The spirit callers could just tap the power of the spiritual realm directly. If an oathkeeper is worth twenty men in battle, a spirit caller was worth a whole army, because they could just call up the spirit of the battlefield for them, and it would lay waste to all their enemies.”