Is it still necessary to use legacy Shopify customer accounts for compatibility with the Bold Subscriptions app, or can we now switch to the new Shopify customer accounts?
Yes, you can switch to the new Shopify customer accounts. More information on this can be found here: https://support.boldcommerce.com/hc/en-us/articles/5831548670356-Shopify-Feature-Compatibility-with-Subscriptions-for-Shopify-Checkout#compatible.