Which version of ZFS allows shrinking of a pool?

I found a list of versions and their Solaris release numbers http://download.oracle.com/docs/cd/E19253-01/819-5461/appendixa-1/index.html

I know that you can grow a pool by replacing drives with larger ones or adding new drives or mirrors to the pool. I heard that ZFS did not yet support shrinking pools by removing drives/mirrors. But that has probably been changed.

Which version (if any) released the ability to shrink a pool?

No current release can shrink a pool and I have not heard any announcements of an upcoming feature to do so. Considering how rare it is that a properly designed pool would need the ability, I wouldn't expect it anytime soon.

The new ZFS version in Solaris 11.4 allows shrinkage of arrays. Say you have 5-disk array, you can now shrink to 4-disk array.

And, dedup has been fixed! It is using Greenbyte superior deduplication.

I cant find that it is supported, yet anyways.

Here's a printout from my Solaris 11 Express running ZFS pool version 31.

zpool upgrade -v
This system is currently running ZFS pool version 31.

The following versions are supported:

VER  DESCRIPTION
---  --------------------------------------------------------
 1   Initial ZFS version
 2   Ditto blocks (replicated metadata)    
 3   Hot spares and double parity RAID-Z    
 4   zpool history    
 5   Compression using the gzip algorithm         
 6   bootfs pool property    
 7   Separate intent log devices    
 8   Delegated administration    
 9   refquota and refreservation properties    
 10  Cache devices    
 11  Improved scrub performance    
 12  Snapshot properties    
 13  snapused property    
 14  passthrough-x aclinherit    
 15  user/group space accounting    
 16  stmf property support    
 17  Triple-parity RAID-Z    
 18  Snapshot user holds    
 19  Log device removal    
 20  Compression using zle (zero-length encoding)    
 21  Deduplication    
 22  Received properties    
 23  Slim ZIL    
 24  System attributes    
 25  Improved scrub stats    
 26  Improved snapshot deletion performance    
 27  Improved snapshot creation performance    
 28  Multiple vdev replacements    
 29  RAID-Z/mirror hybrid allocator   
 30  Encryption    
 31  Improved 'zfs list' performance

For more information on a particular version, including supported releases,
see the ZFS Administration Guide.

It looks that Alex Reece works on implementing the feature in the OpenZFS project: OpenZFS Device Removal blog.

Thorough ZFS in Solaris and OpenZFS are two different projects (see Wikipedia: ZFS).