We do not support Ubuntu Trusty for a long while now, so while that folder might still exist, the repo (and thus also the signature) was not updated since Dropping Ubuntu/trusty builds for Foreman 1.16
You can find old (archived) releases at https://archivedeb.theforeman.org/ (and the signature there is in the process of being re-generated using the updated key), but you’re not getting any updates since 6 years, so please update 