Q. What benefit does removing borders - except the bottom one - have?

No benefit for the User.
Arguably a dis-benefit by creating micro-seconds of doubt, hesitation and micro-stress - versus the near certainty of a fully enclosed box which is consistent with many if not most forms - paper and on-line.

The idea that the brain cannot process 3 extra lines is nonsense. Humans process way more detail every moment of their lives.

Presumably the "benefit" is in getting closer to some designers' "minimalist aesthetic".
Have these designers never tried navigating a snow-covered (near uniform) landscape ?

"What publicly available tests show that is no (bad) impact on users?"
I await the minimalist's answers with interest.