When a DBR version reached End of Support it means that the version will no longer receive security patches and workloads running on that version will no longer be eligible for Databricks support. Unsupported versions may be subject to security vulnerabilities and for that reason should not be used. But clusters that are running the DBR will not necessarily be automatically removed or otherwise made unavailable when they reach the EOS date.
The full details on the policy can be found here in the documentation.
