What linux distribution?
Splunk 6.3.3 is pretty ancient (2015)...
As you can see in the docs, for Linux it mostly depends on your kernel version. Upgrading to a newer version of the linux distribution probably doesn't matter much:
I was hoping that was a typo 🙂
I guess the answer is yes, at least from a basic system requirements perspective. But I don't know of course what exactly changes from 7.4 to 7.6 and whether any of that impacts splunk. Apart from the upgrade procedure itself which may cause temporary downtime which you want to plan carefully and upgrade different splunk hosts (assuming you have a distributed setup) in a smart order to cause minimal impact.