This doesn’t look good.
https://ircbots.debian.net/judd/#judd-builddep
(checkbackport [–fromrelease ] [–torelease ] [–arch ] [–verbose]) – Check that the build-dependencies listed by a package in the release specified as “fromrelease” are satisfiable for in “torelease” for the given host architecture. By default, a backport from unstable to the current stable release and amd64 are used.
checkbackport virtualbox
Backporting package virtualbox in sid→buster/amd64: unsatisfiable build dependencies: Build-Depends: debhelper-compat (= 12).
checkbackport virtualbox --verbose
Backporting package virtualbox in sid→buster/amd64: unsatisfiable build dependencies: Build-Depends: debhelper-compat (= 12).
buster: bzip2,default-jdk,jaxws,dh-python,dkms,docbook-xml,docbook-xsl,dpkg-dev (>= 1.15.6~),g+±multilib,genisoimage,gsoap,acpica-tools,imagemagick,kbuild (>= 1:0.1.9998svn3098~),libasound2-dev,libcap-dev,libcurl4-gnutls-dev,libdevmapper-dev,libdrm-dev,libegl1-mesa-dev,libgl1-mesa-dev,libglu1-mesa-dev,libgsoap-dev,libidl-dev,libopus-dev,libpam0g-dev,libpixman-1-dev,libpng-dev,libpulse-
dev,libqt5x11extras5-dev,libqt5opengl5-dev,qttools5-dev-tools,libsdl1.2-dev,libssl-dev,libvncserver-dev,libvpx-dev,libx11-dev,libxcomposite-dev,libxcursor-dev,libxdamage-dev,libxext-dev,libxi-dev,libxinerama-dev,libxml2-dev,libxml2-utils,libxmu-dev,libxrandr-dev,libxrender-dev,libxslt1-dev,libxt-dev,lsb-release,lynx,makeself,module-assistant,python3-dev,texlive-fonts-extra,texlive-fonts-recommended,texlive- (1 more message)