/private/var/folders not getting empty after reboot on macOS Catalina
It's likely that unneeded files will be cleared if you perform a Safe Boot. And then restart normally.

However, a detailed account of cleaning out /var/folders can be found here,  

http://www.magnusviri.com/Mac/what-is-var-folders.html

which seems to indicate that not everything gets deleted on a Safe boot.

But unless you are desperate for storage space or experiencing some issues, I wouldn't recommend wholesale deletion of the entire contents.