The Linux version source was released, the DOS version source was not...
Here is a nice read: 10 years of Doom - A Slightly Condensed Genealogy of DOOM Source Ports
Also consider that in 1997 there was not much interest in true Mode-X usage. Mode-X with planar buffering is hack to get a speed gain on 386s and early 486s, which would otherwise be unable to transfer the 320x200 pixels at the desired framerates. For Pentiums it is just a unnecessary annoyance for the Developers.
Linear Mode 13h was there already, and became the easier choice. Then Pentiums made VESA modes a viable option for action games as well.
There are four ways I can think of to draw in 320x200 8bpp in DOS:
1-Mode 13h, unfortunately without page flipping.
2-Mode-X with planar image buffering, with or without page flipping.
3-Mode-X while keeping the image buffers linear, and convert things during the screen draws. No speed gain, but does allow for page flipping.
4-VESA mode 320x200 8bpp, with or without page flipping.
MBF 2.04 supports 1+3+4.