Separate GND-plane for microcontroller crystal resonator
First of all you didn't specifiy how many layers your pcb had so I assume you already had a &gt; 4 layer with a complete gnd plane.
Back to your question I checked out Application Note AN2867 you mentioned and found that it's a general design guide for oscilator pcb land for STM microcontroller not a reference design for your specific model so you are free to ignore it.
First note that crystalls are differetial passive devices so ground plane only affect trace impedance but no effect on XTAL itself or the signal. Your concern is right for GND separation only in the case you have had a very sensitive analog signal passing by in adjacent to XTAL.
You may had also intrest in reading &quot;Be Careful with Ground Plane Breaks&quot; part of Staying Well Grounded from analog devices.
So the answer is you didn't had to bother about breaking the plane at all. In my experience unless you didn't have a uVlot signal on your board, ground separation had no impact but to degrade signal integrity and create more issues on pcb