The exhaustion was visible, and productivity was very low. Since everyone vocally thought it was a "waste of time", there was a Fight-club-style "rule" where we couldn't talk about the downsides in public or even in Scrum meetings. We missed several deadlines for an MVP, code quality was abysmal and the product barely worked.
It stopped when the CEO took issue with it. He gathered the team to ask "why is the productivity so low?". There was an awkward moment where a junior dev was trying to answer but a tech lead was trying to tell him "don't break the rule".
It was an absolutely surreal experience, but not out of place in that startup.
EDIT: I also remember one of the tech leads complaining during a Sprint Retro that some of us weren't reading Slack fast enough, and we said "well, I was paring, I wasn't at my computer", to which he became a bit agitated and visibly angry. It was an awful place to work.