It seems to be an experimental feature not officially enabled:
Enabled by default now by TorBirdy 2.3: Our Latest Release of TorBirdy for Thunderbird Includes New Enigmail Features | The Tor Project
Unfortunately, 2.3 didn’t make it into Stretch. It’s a pretty useful fail-safe. Is it worth backporting from Buster in Whonix 14?
It would be cool but we’re lagging behind with maintenance of Whonix 13 as well as Whonix 14 development (stretch based Whonix release still not ready). Quite difficult remaining todo.
After Whonix 14 I want to slow down on features and work on bug fixing, refactoring, automation and tests.