There is no way to restrict UIE based on OS version, etc. You can customize the enrollment screen under Settings>Global>User-initiated Enrollment>Messaging. You can use markdown language to format the message on the screen, including links to knowledge, etc. What we did was then create a smart group for all older OSes that we didn't want enrolled and used that as an exclusion on all profiles and policies so basically nothing would get installed. We also had a policy with a Jamf Helper message scoped only to that smart group for the old OSes that would pop-up at every check-in telling the user they enrolled an old OS and they need to wipe the Mac, upgrade the OS and enroll again.
