We strongly recommend upgrading as otherwise you will be only Self Service support. If you need to raise cases with us you will have to purchase Extended support.
Any fixes or patches are no longer done on old versions.
If you're On Premise customer, please consider migrating to SaaS where we do all the patching and upgrades. Being SAAS has huge benefits in the sense you will not have to plan and do the upgrade yourself, makes things easier for you to stay on current version.