There is a big difference between an LTS release Support date and how long a Distro will be supported.
I think what you are asking about is how long the Releases are supported for.
Zorin OS 15 is supported until April of 2023.
Zorin OS 16 will be supported until April of 2025.
How long the Zorin OS Distro is supported: Indefinitely.
As long as the ZorinGroup remains...