Manual focus lenses where used for many years on SLRs. The only thing different about focusing a manual focus lens on a DSLR is the focusing screen. But Canon offers the old-style focusing screens for certain camera bodies; these show the actual depth of field even at f/1.4. That's all that's need to make the Otus work well on a DSLR.
The resolution of current digital sensors exceeds that of film, particularly when you consider the 'typical' print sized enlarged from 35mm negatives vs. print sizes easily possible with a ≥18 MP digital file. That means slight focus errors that were tolerable with film are often unacceptable with digital.
Also, Canon doesn't really offer 'old-style' focus screens, they offer 'not-so-old-style'. Currently, you can get a screen with without the same degree of laser microetching for brightness - those are the 'super precision matte' screens that show you the true DoF of fast lenses…or
you can get screens with manual focus aids (split prism or microprism) but they're based on the stock screens that don't show the true DoF of fast lenses. When shooting film, there were focus screens that both showed the true DoF of fast lenses and
had the split prism/microprism collar focusing aids.