As far as I know, Debian does not have any strict policy or dates for when they will stop building and publishing OpenJDK 11 binaries out of the jdk11u branch in the OpenJDK project.
The jdk11u branch [1] (the source code) is currently steward by Red Hat, SAP employees and other contributors, and both Red Hat and SAP have a support roadmap, which currently goes until at least 2023 (SAP) [1] and 2024 (Red Hat) [2].
So, Debian is not providing an LTS binary. Debian is producing binaries out of the source code maintained by others. Once jdk11u stops receiving updates, Debian also stops shipping updates, because Debian has no LTS commitment.
The jdk11u branch [1] (the source code) is currently steward by Red Hat, SAP employees and other contributors, and both Red Hat and SAP have a support roadmap, which currently goes until at least 2023 (SAP) [1] and 2024 (Red Hat) [2].
So, Debian is not providing an LTS binary. Debian is producing binaries out of the source code maintained by others. Once jdk11u stops receiving updates, Debian also stops shipping updates, because Debian has no LTS commitment.
[1] https://wiki.openjdk.java.net/display/JDKUpdates/JDK11u
[2] https://blogs.sap.com/2021/07/21/support-extension-of-sapmac...
[3] https://access.redhat.com/articles/1299013