There is a bug report about this: TIFF Export Creates BigTIFF Files With Limited Compatibility Even With Little Images (#8339) · Issues · GNOME / GIMP · GitLab and a future fix.
You can ‘borrow’ and use the old Gimp 2.8 file-tiff-save plugin which does not have all big-tiff options. Good for basic single page files. As this example:
But I do not really recommend it. Wait for the next version. Also I see no real difference in file size, This from ImageMagick (IM)
test-big.tif TIFF64 4408x2856 4408x2856+0+0 8-bit sRGB 48.0248MiB 0.010u 0:00.003
test.tiff TIFF 4408x2856 4408x2856+0+0 8-bit sRGB 48.0246MiB 0.000u 0:00.003
All using kubuntu 20.04 / Gimp 2.10.32 / IM 7.1.0