How do I cancel my legacy contact?

You will find the instructions at the foot of How to add a Legacy Contact for your Apple ID:

You can remove someone as a Legacy Contact at any time in your Apple ID settings. 

• On your iPhone, iPad or iPod touch, go to Settings > [your name] > Password & Security.
• On your Mac, choose Apple menu  > System Preferences, then click Apple ID and select Password & Security.

Under Legacy Contact settings, choose the contact's name to see more options. 

Tap or click Remove Contact to remove them as a Legacy Contact. They won't receive a notification of your decision, but you won't appear in their Legacy Contact list if they have an Apple device that includes this setting. The access key they received when you added them will no longer work.

there are no legacy contact settings in the location mentioned (on my Mac Pro 2020)