There are SQL Server end-of-life dates you should be monitoring. Once a system has reached end-of-life you will no longer receive support from Microsoft, and there will be compliance issues as auditors will have issues with unsupported production systems having potential security issues. You should develop a written plan to upgrade existing systems to a supported version, test the plan to verify you won’t have any issues, and implement the plan before your existing systems reach end-of-life.
This chart from Microsoft will help you understand the end-of-life dates for the various versions of SQL Server.
Summary SQL Server 7 - 1/11/2011 SQL Server 2000 - 4/09/2013 SQL Server 2005 - 4/12/2016 SQL Server 2008 - 7/09/2019 SQL Server 2008 R2 - 7/09/2019 SQL Server 2012 - 7/12/2022 SQL Server 2014 - 7/09/2024