Whonix 14 will certainly be stretch based. There is still a lot TODO for stretch support:
https://phabricator.whonix.org/project/view/60/
I won’t be copying in links to git commits related to stretch support as these are too many in too many repositories. However, one can follow them by having a look on my github profile [and/or by staying on top of Whonix source code changes]. I may generate progress report of this month at the beginning of next month.