This question is in reference to Atlassian Documentation: Atlassian Support End of Life Policy
If the JIRA version has reached the end of support date, is it going to affect the functionality?
What will happen to the existing plugins that are in use?
Will it allow to add new plugins in future?
Nothing changes technically. JIRA continues to work as before, existing add-ons will work as before. New add-ons will work as they're built for it. But you will find add-on authors generally won't update add-ons for unsupported versions, so while an add-on may look as though it's having lots of new stuff added or improved, newer versions won't work on your system.
Atlassian can refuse to support you is the main change - they will use a stock answer of "upgrade to a supported version" if you raise anything (but will generally try to help somewhat, with links to docs about known problems)
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