Are Velo API consistent and Stable?

Talking about consistency:

I need to update the plan when any item is purchased from the Pricing Plans...

I see that API Docs suggests to use the following code in events.js file as a event callback=>

export function wixPricingPlans_onPlanPurchased(event) {
  // ... code here
}

However it's never being called

Then I came across another section in the document that says to use wixPricingPlansBackend_onPlanPurchased instead... I tried to use it as well but --- it neither works...

Talking about Stability:

The question also arises if these API are stable??? And If I start using them in my production application would they work consistently and behave stable?