It would be nice adding some extrepo entries for the main Qgis archive, which is available since
a long time and a lot of releases, to integrate the Qgis version we distribute in main.

The QGIS archive distributes both LTS and latest versions and even a
nightly version for multiple distribution including testing/sid.

Possible minimal entries for the Yaml index could be
---

qgis-latest:
  contact: qgis-developer@lists.osgeo.org
  description: The latest version of the FOSS desktop geospatial application based on Qt
  gpg-key-checksum:
    sha256: 970c8adb8a202ce6a5fbca4939fc148dcc7685c2a2b2ab63fb673e93f9e81217
  gpg-key-file: qgis.asc
  policy: main
  source:
    Architectures: amd64
    Components: main
    Suites: stable
    Types: deb
    URIs: https://qgis.org/debian

qgis-ltr:
  contact: qgis-developer@lists.osgeo.org
  description: The Long Term Release of the FOSS desktop geospatial application based on Qt
  gpg-key-checksum:
    sha256: 970c8adb8a202ce6a5fbca4939fc148dcc7685c2a2b2ab63fb673e93f9e81217
  gpg-key-file: qgis.asc
  policy: main
  source:
    Architectures: amd64
    Components: main
    Suites: stable
    Types: deb
    URIs: https://qgis.org/debian-ltr

---

but more distributions are also available along with nightly alternatives.

More information and releases are here:

https://qgis.org/it/resources/installation-guide/#debian--ubuntu

Find herein enclosed the Qgis current key, which is subject to expiration.