It requires to build a backport. I am downloading virtualbox sources from Debian sid, build the backport Debian packages (deb’s) for Debian buster ad then upload to Whonix repository.
Since the version difference (changelog) VirtualBox 6.1.6 vs VirtualBox 6.1.8 isn’t security relevant and also otherwise brings no changes that I find critical importance, it will take time to upgrade VirtualBox 6.1.6 to VirtualBox 6.1.8 in Whonix repository. Possibly an upgrade will be skipped entirely.
Meanwhile, if you want a newer VirtualBox version anyhow, you’re mostly on your own. This page has some hints:
Not a Whonix issue. The technical background / mess is documented here:
Installing VirtualBox on Debian 10 / Debian buster / buster-backports / fasttrack
Free Support for Whonix ™ applies. Installing a newer version of VirtualBox is unspecific to Whonix. Same way that would work for Debian should also work for Whonix.
Whonix can’t fix all mess. Whonix isn’t Oracle Corporation (the company who bought VirtualBox). Quote wikipedia:
| Revenue |
 US$39.50 billion (2019)[3]
|
|---|---|
| Operating income |
US$13.53 billion (2019)[3]
|
| Net income |
US$11.08 billion (2019)[3]
|
| Total assets |
 US$108.70 billion (2019)[3]
|
| Total equity |
US$22.36 billion (2019)[3]
|
| Number of employees | 136,000 (2019)[4] |