Welcome to the Atlassian Community!
All versions of Jira server will continue to work as they have before. The licence only limits the number of active users you have, it has no effect on functionality!
The end of support simply means they will be unsupported, and not have any effort put into maintaining or improving them.
Patching critical issues will continue for long-term-support, but that's only happening because Atlassian continue to support and develop Data Center, and when patches are needed for that, they are applicable to server installations as well.
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