Welcome to the community!
You can find the EOL for our versions here: Checkmk Versions
For 1.6 it was planned like this:
| Version | Release-Date | End of active maintenance | End of passive maintenance |
|---|---|---|---|
| 1.6.0 | 2019-09-24 | 2021-03-17 | 2022-09-08 |
But we will likely extend the active maintenance by several months due to the longer release cycle of Checkmk 2.0.
And yes, you can use 1.6 clients with Checkmk 2.0, but some functionality will not be possible then. So I recommend updating them as well over time.