TRIM is not supported in the Unraid array, but has always been supported in pools e.g. Cache pool.
ZFS when used in the traditional sense (pools), supports TRIM already.
ZFS can be used in the Unraid array as single disks filesystems, which loses most features people want from ZFS so isn't common, and in this scenario TRIM is not currently supported.
Until a later release where the concept of pools and array gets a shake up, the workaround is to create a token device in the array, such as a usb disk, and utilise a pool for zfs, just make sure you update your share preferences to Cache only so data stays there.
My personal favourite version of the technique is to add a HDD to the array and use this as a backup target.