Does Notion Thief replace card draws from opponents' triggered abilities or only spell effects?

Notion Thief's replacement effect reads: 'If an opponent would draw a card, that player skips that draw and you draw a card instead.' This is a replacement effect (CR 614.1) that modifies any draw event, completely regardless of what caused the draw.

Under CR 120.1, 'A player draws a card by putting the top card of their library into their hand.' Any time this event would occur for an opponent — whether from a spell like Divination, a triggered ability like Howling Mine's upkeep trigger, an activated ability like Arcanis the Omnipotent's tap ability, or a static effect — Notion Thief's replacement effect applies (CR 614.4).

The source of the draw is entirely irrelevant. The replacement effect only cares about who is drawing (an opponent) and that a draw event is occurring. It intercepts the draw at the moment it would happen and substitutes a different event (you draw instead).

Concrete example: Your opponent has a Phyrexian Arena on the battlefield. At their upkeep, Arena's triggered ability triggers and would cause them to draw a card. With Notion Thief on your side, that draw is replaced — your opponent skips the draw and you draw a card instead.

Note that if multiple replacement effects apply to the same draw, the affected player (your opponent) chooses the order of application per CR 616.1, but Notion Thief's effect will still intercept any draw it is applicable to.