Hi Krista,
No, the VirtualBox Extension Pack is not required to run any VM (including BitCurator), as long as you don't need access to USB 2.0 or 3.0 drives, and are you are not running on a host machine using an NVMe SSD drive.
You can easily disable the Extension Pack-requiring USB features in the BitCurator VM by selecting that VM and navigating to Settings->Ports->USB and clicking on "USB 1.1" for the passthrough instead of USB 3.0 in the VirtualBox Manager. Recent BitCurator releases (1.8.12 and 1.8.16) should boot just fine once you've done this. I just tested it!
Hope this helps.
Regards,
Kam