If you like you can close this bug-report. linux-headers-5.10.0-4-amd64 version 5.10.19-1 ships the necessary files and I was able to build virtualbox-dkms kernel-modules:
root# LC_ALL=C ll /lib/modules/5.10.0-4-amd64/updates/dkms/ total 852K drwxr-xr-x 2 root root 4.0K Mar 3 04:00 . drwxr-xr-x 3 root root 4.0K Mar 3 04:00 .. -rw-r--r-- 1 root root 755K Mar 3 04:00 vboxdrv.ko -rw-r--r-- 1 root root 21K Mar 3 04:00 vboxnetadp.ko -rw-r--r-- 1 root root 61K Mar 3 04:00 vboxnetflt.ko - Sedat -
