Terms of service on built-in page?

You're correct, the module doesn't currently handle this. It looks like this is a setting that you add that is specific to the recurly hosted pages configuration. And I'm not seeing anything about it in the Recurly.js docs https://recurly.com/developers/reference/recurly-js/, or any way to get at the setting via the API https://recurly.com/developers/api/v2021-02-25/index.html. So I'm interpreting this as something that should be done in the implementation code (aka Drupal in this case). Maybe via hook_form_alter()?

We could add a setting to the Recurly module that includes a terms of service field and then add that to the form on the Drupal end as an option.