My guess would be that there are No apps that would be affected by a breaking change to remove support for time-only. Such apps would already have encountered the 1899 issues and adjusted. So that change should be OK.
But on -md, I think the time-only is valued, though not required. If we create a task we can assume the time is today, so setting time-only makes sense. However, if there is a new -mt switch for setting the creation time-only, then that covers all options and would be consistent with other switches:
-md = "date" or "date time"
-mt = "time-only"
HTH
T