Linux kernel 3.x and later (32-bit) was deprecated in 6.3 so you cannot upgrade to Splunk ver.6.3.x.
Note: Linux kernel 3.x and later (64-bit) should be supported in Splunk ver.6.3.x.
Could you please expand on why this is/was necessary? I realize this answer is quite old by now, but its always helpful to understand WHY something is recommended.
sleep delay allows PAM to be fully spun up so that ulimit settings are applied properly when Splunk starts. Another alternative and more details are discussed in George Starcher's blog post Splunk Ulimits and You.