Does Notion Thief replace draw triggers from Rhystic Study?

Notion Thief has a replacement effect (using the word 'instead') that applies whenever an opponent would draw a card — it replaces that draw so that opponent draws nothing and you draw a card instead. This is governed by CR 614.1a, which covers effects that replace an event with a different event.

Rhystic Study creates a triggered ability that goes on the stack whenever an opponent casts a spell and they don't pay {1}. That trigger itself is not a draw effect — it merely instructs a player to draw a card when it resolves. Notion Thief does not interact with the trigger going on the stack or resolving; it interacts at the moment the draw would actually occur (CR 120.2, 614.5).

So the sequence matters: the Rhystic Study trigger resolves and says 'you draw a card.' At that precise moment, if the drawing player is an opponent of Notion Thief's controller, Notion Thief's replacement effect intercepts the draw. The opponent draws nothing, and the Notion Thief controller draws a card instead.

Example: Your opponent casts a spell, doesn't pay {1}, and Rhystic Study's trigger resolves telling them to draw a card. Because you control Notion Thief, that draw is replaced: your opponent draws zero cards and you draw one card. The trigger resolved normally — Notion Thief only acted at the draw step itself.

Note: if multiple replacement effects apply to a single draw, the drawing player (or in layered cases, the affected player) chooses the order of application per CR 616.1. But Notion Thief is the only replacement effect here, so it simply applies.