This Tails issue is also relevant to Whonix, because the last commit on the Thunderbird config file where mail.chat.enabled is set to false reads “Updated to reflect Tails version as of 2022” .
Having Thunderbird chat features enabled would give Whonix a default Matrix client.
Remains to be seen in that issue if the Tails team feels that " there’s a reason for not enabling Thunderbird chat features", but I thought Whonix could discuss as well.