There appears to be some backward-compatibility mode iff (and the manual isn't crystal-clear on this) you upgrade from 4.x to 5.x. Then apparently server:/home/user will still work.
There is stuff that provide backwards compatibility, but it's not upgrade-specific; references to "/XXX" are treated as equivalent to "/vol/<root volume>/XXX".
If you upgrade from 4.x to 5.x, you presumably have a 4.x-style "/etc/exports" file, without the "/vol/<volume name>" stuff in it, so what shows up from a "show me your exports list" RPC request to the mount daemon is a set of pathnames without the "/vol/<volume name>" stuff.
However, if you install 5.x from scratch, the exports list created by the setup code has the "/vol/vol0" stuff in it for the root volume, so that shows up in the exports list.
You can undo that in the exports file, and it should work - e.g., you could export "/home" rather than "/vol/vol0/home" (or whatever you call the root volume - you don't have to call it "vol0").