That’s unrelated to the deprecation. His signing key expired and you need to download the fresh one.
Key download mirrors:
- http://www.w5j6stm77zs6652pgsij4awcjeel3eco7kvipheu6mtr623eyyehj4yd.onion/keys/derivative.asc
- https://www.kicksecure.com/keys/derivative.asc
Verify it (fingerprint 916B 8D99 C38E AF5E 8ADC 7A2A 8D66 066A 2EEA CCDA) and then place it in /usr/share/keyrings/derivative.asc in your template or your Standalone.