You can upgrade from Jira 8.4.3 -> 8.17.x without any step upgrades.
Be sure to check out the release and upgrade notes.
See the right column at https://confluence.atlassian.com/jirasoftware/jira-software-release-notes-776821069.html
The upgrade notes will tell you what you need to know technically, e.g. in 8.13.x https://confluence.atlassian.com/jirasoftware/jira-software-8-13-x-upgrade-notes-1018783378.html there are changes to the start up files which may affect any automated deployments.
The release notes will tell you what new features are available in the front end, to end users and admins.
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