Why would you NOT PATCH a Linux Kernel?

This maybe a dumb question, however I have to ask.

The previous system admin left a cryptic note not to patch the Linux Kernel on a few of our legacy RedHat Servers.

There was no reason why, just said not to patch. However there is quite a bit of performance issues with these and we are getting ready to retire them.

However for my own learning, I wanted to see if others have ever heard of this or ran into this and if there is a reason why behind this? The only thing that I can think of is that a vendor may request not to upgrade the kernel due to third party software.

thanks