This might not be new, I hadn't dealt with WordPress/WooCommerce in a while.

Currently (v8.8.2) in a new WooCommerce installation the "Checkout" page is created using blocks, like so:

    
<!-- wp:woocommerce/checkout-payment-block -->
<div class="wp-block-woocommerce-checkout-payment-block"></div>
<!-- /wp:woocommerce/checkout-payment-block -->

(...)

  

The problem

This might introduce breaking changes to the plugins and themes you normally use. For example, I couldn't add a new field - programatically or using a plugin.

The fix

Remove the blocks and revert to using the shortcode:

[woocommerce_checkout]