With #22747 there's now validation on addition of Puppeclasses with the API which also allows use of friendly which is incredibly useful. On deletion however this was omitted (also means you aren't able to ascertain with the API response that you have, in-fact, tried to delete a valid puppetclass).

Should be able to do something like this;

-X DELETE https://foreman/api/v2/hosts/testhost/puppetclass_ids/ntp

or, indeed

-X DELETE https://foreman/api/v2/hosts/testhost/puppetclass_ids/invalid_class
response = Error