This can be installed manually, but it will be nice to have it in Tor Browser by default, as well as having an /usr/bin/update-torbrowser script that is aware of it
I think it would be better if The Tor Project added this to TBB? Write a feature request against TBB?
If only Whonix had it installed by default, users using Whonix Tor Browser in Whonix would end up with a different web fingerprint than other Tor Browser users.
Also lots of other issues would open up. Some from https://www.whonix.org/wiki/Dev/Default_Application_Policy.