David Kimura PRO said about 3 years ago on Payment Gateway Basics with Stripe :
  The Stripe API 2020-08-27 and later no longer have the customer.subscriptions option by default. Instead to get the user's subscription you can do this.

Stripe::Subscription.retrieve(user.stripe_subscription_id)

or

Stripe::Subscription.list(customer: user.stripe_id).first