This is exactly where research needs to go next, but I expect most groups will carry on focusing on the extent to which “volume” is different with different rest periods, neglecting to observe that the volume has no effect outside of acting as a proxy for CNS fatigue.

I would expect it to vary depending on the type of exercise and its aerobic demand. So the threshold for diminishing returns would be longer with squats than with curls, for example. Even so, I doubt 10 minutes would be better than 5 minutes, TBH.

Also, I would expect the ideal rest period to increase with increasing number of sets. A good protocol might actually be something more like 2 minutes between sets 1 and 2, 3 minutes between sets 2 and 3, and then 4 minutes before the next exercise (if performed for the same muscle group). That might match the way in which CNS fatigue accumulates and dissipates better than using the same rest period between each set.