* Add: New Customer Notification feature – sends reminder emails for upcoming subscription renewals, trials ending, and subscription expirations.
* Fix: Prevent adding products to the cart if a subscription renewal is already present.
* Update: Improved performance of wcs_get_subscription() when querying by product and customer or order.
* Update: Improved performance when checking limited subscription product availability.
* Update: Deprecate upgrading from versions of WooCommerce Subscriptions prior to 3.0.0 (released Jan 2020).
* Dev: Minor refactoring of `init` method in `WC_Subscriptions_Upgrader` class.
* Dev: Introduce the filter `woocommerce_subscriptions_synced_first_renewal_payment_timestamp` to enable plugins to modify the first renewal date of synced subscriptions.
* Dev: Update `get_post_meta()` calls to fetch product meta using CRUD getters.
* Dev: Update subscriptions-core to 7.7.1